It depends on the situation. If it is a loaded biker, I always pull over just to say "Hi" even if they are riding along. I usually try to give them a few bucks to buy them a beverage for a fellow tourist. Usually, a biker is a tad nervous but once they realize I am not a threat, they are at ease. If it is a racer-type, I slow down to see if they look like they know what they are doing, i.e. are actively repairing a flat. Like others, I ask if they need anything and is everything OK.
I have passed loaded riders while I was on a car vacation, pulled over and offered them water in Utah in the summer. This was gratefully accepted.
Absolute minimum for all riders is a friendly toot of the horn and a wave.
Once I had a older guy in an old pickup with a confederate flag in the rear window slammed on the brake right after passing me wide and I gave a wave. Scared the heck out of me initially but the guy just wanted to say Hi.
Another good question!