The op amp bias current has to be sourced from the PLL phase detector/charge pump, and so, will cause the PLL to lock with a phase offset. This causes reference spurs. For integer-N loops where the loop bandwidth is as high as 0.1 × Fpfd, this is usually a problem, and so, low input bias currents are required.
For fractional-N loops, the situation is different. Loop bandwidths are a much lower fraction of Fpfd, and so reference spurs are much better suppressed. Also, typically, a deliberate offset current is introduced at the charge pump to make the loop lock with a phase offset to improve the linearity of the charge pump. So, here, bias currents are not important as long as they don't move the phase detector offset significantly.