Either way will work, but finding the factors first will make your steps simpler.

For instance, you can add 1/4 and 1/2 by first converting to a common denominator of 4*2 = 8: 1/4 + 1/2 = 2/8 + 4/8 = 6/8 = 3/4. But if you'd factored first and noted that 2 is a factor of 4, you could have converted to aleastcommon denominator of 4: 1/4 + 1/2 = 1/4 + 2/4 = 3/4.

It works the same way with rational expressions (polynomial fractions).