emmydotnet: I believe the correct answer to your question is this:

The valves need to remain on their seats, not merely be kept from dropping into the cylinder, in order to replace the seals and then replace the springs and retainers, keepers, etc. Having the valve on the seat is imperative so that the full length of the valve stem is available to allow work room when the spring is compressed to do the refitting of retainers and such. Springs only compress a small amount beyond the normal valve lift before coil bind occurs. If the valve drops only one-eight inch onto the piston top at TDC, that one-eight inch becomes very important when holding down the compressor with one hand and fitting the other small bits with the other hand, all while sweat is running into your eyes and colorful curses drift from your lips.

Just a thought from one who has a developed a very broad vocabulary of colorful curses while doing these sorts of things just to save a bit of $$.

Cheers,

230/8