In my class, we almost always add acid to base...
I cannot see a particular "law" for either method. Though if you have NaOH in your burette and you leave it overnight (yes, I'm slow at titration and didn't finish it so left the equipment out) it tends to react with air at the top, form a sediment on the bottom, and completely ruin your experiment.
-> from experience.
Though it might depend on what indicator you use. Phenolphthalein? I find it easier to observe the colour change from clear to pink (ie. adding base to acid) than the reverse. That could be a reason...