A slight top up should not hurt.
But I remember there is a "DOT Number" you have to look out for.
ie. DOT3, DOT4, DOT5, DOT5.1
This guy, who made the webpage explaining the DOT types is Not an expert but gives you an idea from his past experiences with car enthusiast.
- http://au.geocities.com/ozbrick850/brake-fluidtypes.html
But not to get paranoid, just double check with you mechanic and again, a slight top up will not hurt.