I recently bought a Canon 1000d DSLR. I'm just an amateur and don't use my camera daily. I use it once or twice a week. Is it okay if I leave the batteries in the camera during this idle time?
Leaving the batteries in your camera for a week or two between uses will be no problem. If you plan to leave your camera unused for six months to a year, you might want to take them out.
And, the lithium battery pack used in your camera (like most current dSLRs) should hold its charge on the shelf fairly well, so you won't have to worry too much about it being drained just sitting there.
(If you do need to shelve it for a long time, the battery will be best if stored at a little less than half full.)
A few references for further reading: Tektronix's Lithium Ion Battery Maintenance Guidelines and Ars Technica on What is the best way to use a Li-ion battery?.
Some rare cameras may have problems where the battery is drained by the camera even when the camera is off. You'll know if you have one of these by whether your batteries still work after two weeks. In that case, there's still unlikely to be any actual harm; battery life might be shortened somewhat, but not more than it would be by actual heavy use.
As long as the camera is fully off, and the battery is not flat.
It is bad for Lithium batteries to be left unused for a long time (more than a few days) with the battery discharged. That's why when you buy something new with a Lithium battery, it always has a little bit of charge on it. In normal use the batteries prefer to stay topped up rather than run right down (that's an old myth).
If you were leaving it for, say, 6 months, apparently it's best for their longevity if you leave them at about 40% charge or a little more. I would also store it without the batteries in the camera. It's very unlikely, but you wouldn't want a battery fault (or a buildup of humidity or condensation) to cause it to leak, melt, etc in your camera. That's extremely unlikely to happen though, I probably shouldn't have suggested it.
The Canon EOS 1000D manual says this on page 12:
If the camera will not be used for an extended period, remove the battery and store the camera in a cool, dry, well-ventilated location. Even while the camera is in storage, press the shutter button a few times once in a while to check that the camera is still working.
It doesn't elaborate on what "an extended period" might be, but given the advice to "press the shutter button a few times once in a while", I think it's safe to assume it means months or at least weeks. It's certainly safe to leave the camera with the battery installed for a week at a time.
A word of caution: You can bet that the advice in the 1000D manual is predicated on the assumption that you're using a genuine Canon battery. Canon currently has a campaign on its web site against counterfeit batteries (and other accessories). Not all aftermarket batteries are "counterfeit" -- there are a number of manufacturers that label their batteries with their own name. Nevertheless, be careful about the quality of batteries that you buy.
I have had multiple rechargable batteries for multiple Canon SLR's for the last 25 years. I keep one (or 2 depending on the body) in them, I also make sure they have more than 50% charge in them when stored. (I usually charge them before storage) For both my pro and amature cameras I have had no issues with either. Different battery types do have different characteristics. The LP-E5 that your camera uses is most similar to the LP-E6s I have, those and the older BP-511s I use have not had any issues storing them in the cameras for 8 to 10 months.
You want to make sure that you charge your batteries before you go out shooting, the battery meter can show higher than what the battery really has for power, if it has been sitting without load on it for months. You start shooting then suddenly you see that the battery levels are critical.
I have a 1000D and from experience I can tell that after 3 months of not using the camera and the battery left inside, the battery monitoring circuitry still runs as long as the battery is inside. And that burns some charge. If you shoot at least once a week, leave the battery inside, if its more than a month. Make sure it stays outside.
As a generic answer I'd suggest NO. Different battery types will behave differently to this. I've seen my electronics destroy with AA batteries because of this. You may be using a battery that will cause it.
There are components in camera that use a very little percent of battery even if its switched off. Take a case of a Li-Ion battery and if these components bring down the battery close to zero percent charge, it affects the battery life and may just kill it.
Also there can be several random things that happen and will create problems with your camera. For instance, while you have left the camera somewhere be and your cat spills water on it. Leaving the battery in will certainly destroy the circuits. If you keep the battery out, your camera will be saved. Consider humidity too. If weather changes drastically, water could condense and damage it.
So for a long time, please dont leave the batteries in.