I will assume that you are increasing the preload on the wastegate actuator arm? I have a funny wastegate on my current turbo that has an adjustable arm, so I can't figure out where some washers would go.
If you are preloading the arm more, you could be limiting the wastegate flapper which in turn could cause boost creep issues in higher gears with sustained pulls. (But then again, it might not - depending on how much preload you have.)
The preloading is probably helping to prevent the wastegate from being blown open by exhaust gasses. (I seem to remember you getting a turbine housing with a larger passage and flapper?)
Personally, if everything works well, you don't have excessive boost creep, and no knock - why worry?

-Kevin