The lower the BB the more it matters. However, as long as you round down and up with the same frequency it shouldn't matter. Although, I like to pretty precise with my bets in certain situations so I definitely use the cents. For instance, if the pot is $3 and I believe that my bluff will work at least 1 time in 3, I'll bet $1.5 so that in the long run I'll do no worse than even on it.