A lot of women do find it's easiest first thing in the morning. Simply because baby has fed less frequently overnight (hopefully!) so you're fuller. As they feed mroe frequently during the day you may find it harder to get 'excess' to pump. (you're actually producing more the more often you empty your breasts, so supply isn't dropping, it's rising, but baby is drinking it all so you feel like you're makign less)
ITs' a brilliantly simple system - baby feeds, you make. If you pump, you'll make more. If your supply has settled down now, you may find you can't get much at first. It can take a while for the 'make more' signals to get through. Also pumping itslef can be much less effective at getting milk out. (It's often easier early on when you have an oversupply)
So anyway, the answer is, do it whenever it suits you.