Tricky subject this. In theory at least, too small bars for the stem will result in major slippage, but if the bars are too big, you'll crush them, thus weakening them and risking a very painful breakage. But in reality, as long as you're carefull (no big bar-to-road imapcts, don't pull/push to hard on the bars when hammering), you can get away with it. Most stem/bar manufacturers only recommend you use their bars/stems, so the vast majority of us are running the same risk (lessened somewhat though). I say go for it.

*Please don't sue me if you crash and die because I told you it was ok to do it.