I've read somewhere that deleting unwanted pictures from a memory card in camera is risky because there's a slight probability that the card will become corrupted, and all the data will be lost. The advice was to keep all the pictures, and delete them all only after downloading them to a computer.
Is this a valid concern?
Unlikely but possible. Nothing you should need to think about.
Every write operation on the card is risky and could trigger corruption if something nasty happens exactly in the moment of the write operation. Deleting a photo is a write operation. The more of those happen, the higher is the risk. Obviously moving the deletion operations AFTER a backup minimizes the risk of data loss. You still have the image saving write operations but are on the safe side with the deletion write operations. So technically the advice is valid.
But the base risk is so minimal that it doesn't matter. The cards (at least common SD cards) are quasi self-contained; the operation works or it doesn't. If it doesn't then the whole card is broken and will probably fail at the normal image saving write operation; don't forget that the only avoidable extra risk is in the deletion operations, a tiny fraction of the operations you do on the card. You shouldn't use the cheapest cards or cards that already showed flaws. With a good quality card the base risk of write operations to fail is so minimal that the additional risk of the deletion operation statistically will not realize. I can't put it better than this XKCD does: http://xkcd.com/1252/
The problem is to argue with someone who insists on the correctness of this advice. It is technically correct, even if borderline insignificant, but the suggested action is not useful. Social advice here: If they want to, let them. It doesn't harm anybody, it doesn't harm them. The "only" negative outcome would be a photo not taken because of a full card. This happens more often than the technical failure on deletion. The negative outcome outweighs the benefits so you shouldn't follow the advice yourself.
No! I don't know where did you read it, but there is no reason why deleting a specific unwanted photo will also trigger a deletion of other photos or corrupt the card.
Bugs can occur at any time and usually for the end user (of the card) it will be very unpredictable, it can happened when you're copying files to you computer, taking new photos, deleting photos or performing any other action, but this is very very rare.
I would add that even if your card had been corrupted from a single short action (such as deleting a photo) you would probably be able to restore most of the information on the card or even all of it.
There's a slight probability of something going wrong whenever you write to a card something will go wrong; the answer by @his covers this nicely, and in sum, it's not worth worrying about. There are, however, two reasons you might be legitimately hesitant.
First, the user interface on cameras is sometimes a bit clunky, and if you're in the habit of deleting files on the camera, it might be easy to accidentally delete a frame you want to keep. I've done this, and have since decided to avoid deleting in camera whenever possible — just swap memory cards when one is full, and sort them at home.
Second, if you delete files from the card, that leaves a logical hole in the data on the card, and future files may only partially fit in the hole, leading to file fragmentation, which isn't a big deal but will reduce your chances of recovering files if you need to run an undelete utility.
So overall, there's no real worry about corrupting the card, but avoiding deleting in-camera may reduce your risk of user error.
The biggest concern deleting photos from the card is that it can be hard to evaluate if it will turn out good on the computer or not. I find the photos always look disappointing on the LCD screen, but they are fine on the computer. Ofc if it is a random shot into nothing (the floor) or seriously motion blurred it is certain that it wont work, and it can safely be deleted, if you are running out of space and dont have an extra card. It is easier to wait though for your selection process.
File system corruption can happen at any write operation (normally not a read operation). Snapping and deleting, so I guess that a 1:1000000 chance of corruption is more likely to happen , the more you do writes and unnecessary deletion is yet another chance to corrupt it. It is worse not to get the perfect shot because it is full though. So I'd fill the card and then delete the most obvious flukes, to get through the session, but even better it is to have an extra card. And then copy all, delete the flukes and erase the entire card.
