What level do I really need? With the new rules I don't know exactly what skill level are required and what are just wasted. I was thinking of adding some fighter or wizard levels to get a more useful character.

I have... er... had a level 1 rogue/high level mage. High level rouges get cool abilities like evade but more than one level isn't nescesary. you will ge the same amount of stat points ( pick pockets etc) a rogue would get but being able to cast spells as well is very valuable.

Both fighter/rogue and wizad/rogue is good picks, id say favor the fighter/rogue though, just since most dont like to multiclass their spellcasters to much.

Some people can do with level 1 rogue, others level 2 or 4 or 5. whatever fits you the most. mixing in level 4 fighter and try to aim for level 10 rogue for some of the rogue's special feat selection is a fun way of doing it.

If you want the powergamer solution ? level 1 rogue and go for wizard (with a drow as race).

Technically, you don't need even a single Rogue level.
Since practically all skills are available at all level-ups, putting points in Disable Device, Open Lock, Pick Pocket, Search, Hide, Move Silently is possible even when not taking Rogue levels.

However taking 2 rogue levels for the evasion skill is the general idea.
Over and above that, further levels are only recommended if you intend to use that backstabbing ability.

You actually don't need very many rogue levels in this game. There aren't that many tricky locks or deadly traps compared to other games.

I guess a better question would have been at what level is it not worth adding more levels? At level 10 you can get greater evasion so would that be the best time to mulitclass. I was thinking of adding fighter levels at that point so he will be able to stand in a fight longer. I have a wizard, a cleric and a bard who I plan on mulitclassing to Sorcerer after level 11 so I don't reall need more spellcasters.

L10 should be enough for a fighter/thief. If you want better fighting capabilities, you could take 2-3 fighter levels early on to increase your BAB and weapon and armour selection.

Go for a Rogue 10 / Mage 20, and a drow.

It's a good mix, just check out Beren's IWD II guide!