During the winter in AZ, CA etc. I never use pay campsites...there's too many free places to stay on BLM and NF lands and usually enough sun that I don't have to break out the gennies to charge the battery bank. Oh and cheap showers in town when I tire of the solar shower.
While I'm on the road I generally check in to a paid campground about once a week so I can enjoy endless hot running water...lol. Whether I take a FHU site or a primitive one depends on how much driving I've done that day - do my batteries need to be put on the battery charger or am I good to go. If I'm staying more than a day so I can do some sightseeing or just to take a day of rest then I'll usually take a FHU site if it's not too expensive, otherwise I'll run the generators.
I try really hard not to have to use the portable generators in the middle of a WalMart parking lot but if I'm staying in a Casino lot, that's different. I've been in a Casino lot the last two nights and I don't think I've seen a TT *without* a gennie set up, there's 3 within eyballing distance running right now...
I use freecampsites.net, allstays.com and casinocamper.com to source out the free spots to stay.
Here in Canada it's a little bit more difficult to find absolutely free campsites but I've found that a lot of the small towns have either town campgrounds or the Lions Club owns a campground...usually quite inexpensive.
I also pay for a campsite when it's someplace that's on my 'to do' list. I cringed when I had to pay $36.00 for the night in Dinosaur Provincial Park but it was ever so worth it. Same with Waterton NP. I'm headed for Banff and Jasper NPs and will have to pay for campsites along the way because there is no other choice.