Is it safe to turn on sprinklers in spring and just drain backflow preventer if freeze is pending?
I can't see a problem as long as you can drain the system. I installed my own sprinklers and the system is pretty unusual because I have very little lawn, mostly shrubs. I do have some low spots that don't drain properly that are a problem. However , as long as there are no undrained areas you should be good. One related problem; I tried to use a hose in cold weather but the water froze before it could get to the end of the hose . This could be a problem for you if you tried to water while the ground was frozen to a depth below your sprinkler system.