How risky is it to share a .blend file?
In general not very risky, meaning, as your question implies, how much Personally Identifiable Information (PII) you would expose if you transmitted a copy of a blend file to a third party, such as, for instance, uploading it to blend-exchange.
Note that in the US, PII has a specific legal definition "Any representation of information that permits the identity of an individual to whom the information applies to be reasonably inferred by either direct or indirect means."
Other jurisdictions have similar legal concepts but with slightly different definitions.
Does it hold a variable some where with your computer name?
Not in general. While it is sometimes possible to infer a computer name from the paths that are sometimes stored in a Blend file, it is usually not included in a path.
However, it might be possible to deduce a user name from a path, because most operating systems use the user's username in the path to the 'home' directory. If you worry about such things, select a user name that doesn't identify you.
If Blender is installed through Steam does it hold your Steam username as a variable somewhere in your blend files?
No. That's not how Steam keeps track of your account information. It would require that they modify Blender to add that ability, and that's not in their interest.
However there is one potential source of risk: add-ons. I know of at least two add-ons (Blender Cloud ID, and Blenderkit) that keep copies of your account credentials so that you don't have to manually log in each time you use them. There are probably others.
Well written add-ons will keep that information in separate files that the add-on accesses so that it's never stored in the blend file; but there's nothing to keep a poorly written add-on from also keeping the information as plain text in text data block in the blend file.
Nor is there anything to keep you from doing the same thing yourself.
You can reduce risk by storing your blender files on a path that's not part of your 'home' directory, and by avoiding add-ons that use login credentials, but it's a pretty low risk to begin with.