Why do you want to shave the head? Is it warped? You want more compression?
How much you shave depends on what your goal is. How much you CAN shave depends on the stock piston to valve clearance and piston squish band. On a stock engine it's pretty pointless unless the head is warped. Might get you half a point in compression before you have to get serious about checking piston to valve clearance or modify the piston so it doesn't run itself into the cylinder head. It will also add slack to the cam chain and retard the cam timing. I've built quite a few honda twin race engines and have yet to shave a head.
Might want to try just going to a thinner head gasket. Stock is about .049"...you could have Cometic make you a .043" gasket and bump compression about 1/3 point...although you wouldn't notice any difference in the way it runs. If you have the clearance you could try something real thin if you can find it...like .030"...which might make a noticable increase in compression.
I was looking for more compression. I understand how engines work, and can fix them, but when it comes to modifying this or that, im lost. The compression ratio on the bike right now is 9.3 to 1, i was thinking of something like 10 or so to 1 would be nice. Are pistons the best way of doing this? The bike is for street use only, mabye I should leave the compression ratio alone?
The Honda twins are kind of hard to get compression out of without doing some pretty radical modifications...race pistons etc. If you are looking for additional performance you'd probably do better going with exhaust and carb modifications, mild camshaft, some cylinder head porting. And of course the big performance enhancer...weight reduction.
You'd get far more hp out of these mods than you would out of increased compression. A few well choosen mods and you could probably up hp by 5-10. Drop off 30 lbs of stock street junk and you'd really start to feel the difference.