ok this has always bothering me and I guess it's been long enough to ask the question.

So 3d sketch dimensioning:

If I'm doing 3d sketch inside assembly or part (ie..with something already there)

The only way to put that 50 dimension is to click LMB somewhere outside that isn't part of existing model - if not that it will pick it for selection of dimension.

Is there a way to click hold something (like ALT in so it skips automatic snap).

This is only a case when adding dimension for single line LENGTH.