At the moment we don't do any since M is full time undies. But before then we only changed her when out if she was poopy or if we were gone a long while and she was drinking a lot. Besides that we would usually wait till we got home.
We usually aren't out very long so we just wait till we get home. He rarely poops (about every 2-3 days) and usually he spends some time working on it at home ;-) so I haven't had to worry about a poopy diaper while we're out yet. On the chance that we are out a while (more than 3 or so hours) we will do a quick change.
It depends on where we are, how close we are to done, and how far we are from home .
I've started scoping out the restroom the first thing I do when we go to a restaurant. I clearly have PTSD (not real PTSD, just, don't want to do that again) from a couple of long, poopy, screaming diaper changes in restaurants/coffee shops with either a single unisex bathroom, or a men's bathroom for one with nowhere to change him but the floor.
I change him as often as necessary. There have been numerous stores that I have gone into that don't have changing tables in the bathrooms. That doesn't even phase me anymore.
I think I did most of my changes while out running errands in the back of my Tahoe lol I felt it was cleaner than any changing table in a bathroom and I wasn't having to search for a place when she was little.