Although the vast majority of upgrades go just fine, backups need to be keep anyway, so why not protect yourself by doing it right?
- Clone the boot volume to an external drive. Make a 2nd clone as a backup.
- Boot the Mac from the external clone.
- Install Lion onto the external clone.
- Reboot, then clone back over the internal volume.
See also How to Upgrade Your System/Boot Drive.
Installing on more than one Mac
Once the installer is run, it deletes the installer app.
Download once, then before installing, copy the “Install OS X Lion.app” from your Applications folder to another drive. That copy can then be used as often as needed for your other Macs.
Installing Mac OS X Lion on a system with a software striped RAID boot drive is reported to destroy the boot volume, rendering it unusable. I have two (2) credible reports of this problem.
I have four other reports that installation on a software RAID boot volume has worked without difficult, so perhaps there is a risk factor that will eventually emerge.