serif fonts (i.e. Times) are good for small print, the kind you see on smaller books. They are usually around 8pt. For your thesis, you will be required to type in larger sizes like 10-12 pt. So Arial (a san-serif font) is good enough to be legible, readable and tidy. You don't need to use serif fonts unless your University guidelines say so. I usually write in Lucida Sans, but for my thesis I might have to convert to Arial (which is exact copy of Helvetica, if you are a Mac user by any chance).