Yeah, like Al said, there are no real rules, unless the sticking is written out. If it is a written exercise or ensemble piece, you'd want to be able to play it as written. On the drum kit, being able to play flams either way gives you greater fluidity and voicing flexibility.
both are valid- usage of one or the other is decided on - upon notation-sticking, execution, hands- prior- is how I think I remember something about- it +/-
in other words relative to what came before it..and what comes after..
if the flam is the second note in a paradiddle for ex.ample,
....I think.......practice both......you will need both on your sticking patterns....cause they be complex and you gots to come out with the proper hand for the next part......could be left or right....be ready.
If you're playing a written snare drum solo it will be whatever makes sense in context. Following a rudimental sticking in a rudimental solo, or whatever makes sense in a concert snare drum solo-- often alternating, sometimes not.
If it's in a band or orchestra piece, or concert snare drum solo, it will be whatever's easiest for you to execute precisely.
If it's on drum set it's whatever you want. I favor left handed flams (rL) on the drum set.