A lot! I have Hughes 1.6 rockers on my motor, took a long time to get it right. A lot of on and off with the rockers and checking with feeler gauges. It's not super critical to get them exactly centered over the valve tip but you have to watch for binding so don't make them too tight.
Using the PVC pipe they ship with the arms helps to keep things in order.
Geometry is a whole other issue that can get pretty involved. You can leave some power on the table if you're way off. I would take the time to check it just to see if it's close. I spent a LOT of time on this. I checked the wipe pattern and it was good. I installed the rocker shims to see if it was different/better and it wasn't. YRMV so don't take my case as fact.
If you can read through this page without your head exploding then you will have a better idea of what you're looking for with rocker arm geometry.
