If you don't compress the cylinder and put the piston at TDC before you remove the valve keepers, the valve will drop into the combustion chamber and you will have to remove the head to fish it out. Probably not much fun.

You can actually get away with just putting the piston at TDC but you will notice the valve drop a little bit when the keepers are removed. It makes it a little more difficult to get the valve springs back on.

What are you using for a valve spring compressor? What are you going to be pulling the seals off with?