Getting a valvejob ensures a new fresh seal for your valves. If your rebuilding your engine you should definately get a valve job IMO. It would be like putting the same pistons or even rings back in the block during rebuild...
The angles theyre talking about doesnt really affect the seal of the valve to the head, its more for better airflow into the cylinder which means more power.

Imagine your driving and you need to take a left at the next intersection.You must slow down substantially to take that 90 degree turn.But say instead of a 90 degree turn, you had TWO 45 degree turns, You could take the turn faster.
The air behaves the same way,smooth transitions...If each valve flows just 10% better and you have 16 valves,think about how much itll help the BIGGEST restriction in your intake!
You may not NEED new valves unless theyre bent/burnt/etc.
I think your head already has a 3 angle from the factory. Your machine shop should know the degree that the seats need to be cut, dont worry about it.Theyll polly cut them close to factory spec I imagine.
And dont forget to have your valvespring installed height and seat pressures measured and adjusted. These can change with valvejobs slightly.
Sorry cant recommend any good machine shops in the area. I havent delt with any yet. My advice is to ask around and get there general reputation. Should cost you around $125.
Good Luck!