If it's just sometimes, and she's still sitting/lying normally, I would guess that it is OK. Cats do some weird things and sleep in some unusual positions.
I did post a thread a month or so ago about Zoey. She had just been very, very ill, and then refused to lie down. She would kind of crunch up on all 4s, and doze off that way sometimes. I later found out that she had a broken tail, which was causing her a lot of pain. "Crunching up" can be a sign of pain. Someone posted about the "meatloaf position" in relation to CRF. Go to this website: http://www.felinecrf.org/symptoms.htm#miscellaneous
and go down to #8 (Crashing) for more info. It wasn't the exact position that Zoey was sleeping in (she was more standing up on her hind legs and crunching her body over).
Like it says, lots of cats rest this way normally. Unless there are other signs of illness, it's probably nothing to be concerned about!