I'd guess that 80% of my flying is done at locations that are not recognized model airfields. You can fly anywhere you want as long as you do so in compliance with the AMA safety code. Be aware that .40 - .60 size models take a lot more room to fly than you might think. Also, as mentioned, flying in an area open to the public generates a lot of curiosity. First thing you know, there are 10 people standing on what you had intended to use as a runway.[X(] Remember these two words when judging a flying site : reasonable, prudent.
Have fun!