My preference is to replace any shutoff valve in a cabinet that is over 20 years old anytime I replace a faucet.
I like to use the 1/4 turns for replacement.
The old shutoffs tend to develop leaks around the stem.
Not every customer wants to spend the extra for that, and it's not necessary all the time. But I do it on my own stuff.
To answer the question above, I would slide a plastic tray near the wall, below the valves.
That way you can pull it out and dispose of any water that is collected.