I prefer to edit photos in Photoshop. However, my photos nowadays are all raw. So, when I open them, Photoshop forces me first into Adobe Camera Raw. It automatically adjusts various aspects of the image. For example, the tint slider is never at 0. For the image I'm working on now, it's set to +16. Does that mean Camera Raw is adding a red tint to the image? Or does it mean the original image has a red tint to it? Likewise, it always defaults to sharpening the image. I'm thinking of just setting all the sliders to 0 and then clicking "Open image" to do my editing in Photoshop. Is there any disadvantage to doing that?
However, my photos nowadays are all raw. So, when I open them, Photoshop forces me first into Adobe Camera Raw.
That's because the information in a raw image file must be interpreted before it can be converted to an image that is anything like what you expect to see on your screen.
Why Use Adobe Camera Raw?
There is no such thing as "The" raw image, there are only possible images that can be produced using the information in the raw file, which is a set of single linear, monochromatic luminance values for each of the photosites (a/k/a sensels, pixel wells, etc.) on the sensor.
Demosaicing to account for the differences due to color filters in the Bayer mask (even when producing a B&W image - remember how color filters altered the brightness of different things by different amounts when we used them on our cameras with B&W film?), gamma correction to convert the linear values to the logarithmic ones we need so that the image does not look more or less like solid black, color temperature, white balance correction, etc. must all be applied to the raw sensor data before it begins to look anything like the thing we took a picture of.
That is what ACR, or any other raw conversion application must do: convert the filtered monochrome luminance values for each sensel into a viewable image by applying color demosaicing, gamma correction, color temperature and white balance correction for various types of light sources, etc. It has to use some value for each of those setting to do so.
If you center everything to zero, you're basically telling Adobe Camera Raw that the photo was shot and properly exposed under more or less ideal conditions: Sunlight with a color temperature of around 5200K to 5500K, no harsh shadows, etc. ACR will apply default gamma curves, color channel multipliers, contrast adjustments, etc. optimized for those assumed conditions. If your light source(s) was(were) significantly different than what the default assumes, then those setting will not be correct to accurately reproduce the color of the scene.
That case means you are applying +16 Tint (which is Magenta instead of Red).
I was not aware that Auto would ever adjust White Balance though. Just the Tonal things, I think. But Adobe Daylight or Cloudy choice does set +10 Tint.
You can double click the button slider on the controls, and they return to default, usually 0.
If you don't want the auto adjustments, you do have an option in Adobe Bridge, menu Edit - Camera Raw Preferences.
Uncheck Apply Auto Tone Adjustments.
We really ought to see the Before case, to know what exposure we are doing with the camera. But then if desired, you can still do this Auto stuff for the current picture in ACR by clicking the Auto link.
Frankly, IMO, most adjustments in ACR, especially White Balance and Exposure, are much better and much easier than in Photoshop.
For the image I'm working on now, it's set to +16. Does that mean Camera Raw is adding a red tint to the image?
This means that the captured data's white balance is shifted towards the magenta. Any image you open will have both temperature and tint at the settings they were at when the data was captured (notice I'm not saying image-- See Michael Clark's answer for why).
As far as I'm aware, the only thing that Adobe changes automatically in Camera Raw is the Sharpening that you mentioned. If you want this to be turned off, you can change the Camera RAW defaults. Usually, I don't mind the minimal sharpening that is applied, but that is a personal decision.