I know how I feel when I am doing something to my wife's computer if there is even the slightest glitch. I cringe to think of the consequences of a serious problem in the process. But, as I said, I have migrated several Macs using Migration Assistant and all went well. Be sure the volume structure on your source volume(s) and your target volume is healthy, you have a reliable source of power for both Macs (I use a UPS on everything), don't get over anxious (migration can sometimes take a very long time -- hours even) and you should have no trouble, so don't go borrowing any.
PPC apps should still run under Rosetta but that is not necessarily true for drivers. Going from PPC to Intel may "break" some printer and other drivers, but if new drivers for the device are not included in the install, you should be able to find universal updates online.
To reiterate: any Tiger versions of utilities such as Cocktail, OnyX. TinkerTool, TinkerTool Pro, Disk Utility, Diskwarrior, Drive Genius, TechTool Pro, TechTool Deluxe, et. al. can and will damage Leopard and Leopard volume structures. Be absolutely certain not to use any of those until they are updated to the Leopard version. Tiger GUI modifiers, application enhancers, Safari plug-ins, etc. won't break the OS but they can prevent applications from running so those too must be updated. FWIW Leopard broke more Tiger stuff than Snow Leopard broke Leopard stuff.
