Good use of fonts can way make your program more visually appealing, but when not used well, can make it ugly, hard to read, or worse. Stick to the plain, bold, and fixed system fonts to maintain some general visual consistency across the operating system. It is perfectly acceptable to use different font sizes, but do it only when you need a heading or something similar and try to limit the number of different sizes to just a few. A window which has lots of different font styles and sizes is harder to read and looks unprofessional. Control labels should always be in the system plain font and size specified by the system.
When calculating the layout for the controls, be sure that you do not depend on the system font being set to a particular font size. Most BControl-derived controls implement the methods GetPreferredSize and ResizeToPreferred to take font size into account. A basic idea of the line height for a font can be calculated by calling BFont::GetHeight() and adding together the values of the font_height structure.
Lastly, do not depend on the user having a particular font installed on the system unless you are willing to ensure that the user has it installed on the system. Font handling for your program is your responsibility, not the user's. If you do follow this route, be sure to check to see if you have redistribution rights for the font's license for the font in question.