It's time to run: It will find more packages to recompile!
In that window you are root to everything and can edit anything. If you need help figuring out what options are available in the then refer to the Gentoo Handbook and Wiki often.
My feelings are this: no you don't need to, but SL 3.3 makes it a piece of cake to world update.
Pre 3.3 I don't recommend, as you will be forever swamped with recompiling.
Now with the world update you are going to run into bumps and this is where reading the ERROR MESSAGES is going to come to be Very Important.
If you listen to what portage is telling you and use the Gentoo bugzilla and Gentoo forums for searching you can usually find the solution.