You can place a spacer on top of the spring but it's always better to install a spring with the correct height in the first place than add a spacer to its top to give you how much lift you want.
I have a spacer on top of my front springs I use for leveling. I don't see a problem adding spacers for lift, but as @Jerry Bransford said, the correct length springs is always preferable.
Spacers are generally fine as long as they don't create the need for bump stop extension to prevent coil bind. Though, it is best to get all or most of the desired ride height from the springs.