A beard is fine as long as it is:
Conservative, i.e. longer than stubble but still short enough to show you are grooming every day.
VERY neatly trimmed
Most importantly the beard can't go too far down the neck, and the neck portion should be equally neat/trimmed.
This isn't my opinion....it's the lecture we got from our Dean of Students during orientation week this year about how to present ourselves in the clinic around patients. Beards are clearly in style now so if you really like it you shouldn't be afraid to have one.
All that being said, clean-shaven is always the way to go if you want to be 100% sure.