Yes, you can assemble them. You'll need to check the assembled valve height to make sure you'll have seat pressure from the valve springs, or the opposite, not enough height, which could result in valve spring bind. Are you having them install hardened valve seats, or touching up your seats? In any case, the new valves need to be lapped in.
Make sure your machinist checks for any sign of valve recession if you're not having new seats installed. You'll also need to have your machinist check valve guide clearance. You don't want to get your heads home and have sloppy stems. Also have him surface the heads so they're nice and flat. And, talking about flat, if you don't have to bore the engine and take it to the machinist, run a straight edge around on the block surface to make sure it is flat.
You'll also want your machinist to machine the guides for real seals.