Yes. It is OK to use larger wire than the code requires. Some times it is even necessary for long runs to avoid voltage drop.

It is however not good pactice as someone might see the 12 guage and think OH I can change the breaker to 20 amp.

One inspector told me that he'd probably fail a circuit using 12/2 for a 15 A for exactly the reason joed mentions. He didn't at all like the idea of mixing gauges in one circuit.

My question is why do you want to use 12/2? If it's just because you have it laying around, I'd just bite the bullet and buy some 14/2 - it's cheap enough. If it's a long run, then I'd at least mark the wire at the end with a tag saying that it's a 15A circuit.