[Nerull]

Just a few points:

1. Monk is one of the worst classes to multiclass, as you get strong abilities all throughout your progression (levels 1-20 all have some good abilities), just like spellcasting classes. If you don't mind losing stuff like quivering palm, spell resistance, and damage reduction, go right ahead and multiclass at 10th level. Personally I wouldn't (but it is your character).
2. Unless you are going to be adding levels in your race's favorite class (or a prestige class), get ready for an xp penalty of 20%, since your monk class level will not be within one of the other class level.
3. Though I like the new multiclassing rules (make more sense), I never really do so myself. I agree with andrewas that you lose more than you gain, especially by multiclassing spellcasters. However, prestige classes are a different story; if you really like shadowdancers, and want to add levels in shadowdancer to your monk, go for it. In most cases, if you are taking a prestige class, you decided long before on a character concept. As far as I am concerned, sticking to a concept leads to a more memorable character that you will be more excited to play.