I always upgrade to the immediate next version. A few times I've gone through two upgrades in a few hours. To go from 8.04 to 10.04 will take a while to do that way, though, and you'll be downloading maybe 1.5Gb of files along the way.
If the GUI upgrader lets you go straight to 10.04, I'd say it can probably be trusted. I've not seen it botch an upgrade.
Wade.