As long as the font is licensed for commercial work, then it is okay to use it. With free fonts, you have to go on a trial-by-trial basis. Some font authors strictly prohibit you from using it for commercial uses. Some want royalties. Some only want you to ask permission first. If you do download freeware fonts, look in the readme file to see what their author allows.
Or, if you have ever purchased a commercial font (such as
this one from Adobe, as an example), most of these are licensed for commercial use, as long as you don't actually redistribute the actual font file. (I picked Myriad Pro, because that's the font used by Apple for all their headlines and marketing, which is obviously commercial uses.)