Sane default for Altium pastemask expansion?
The actual values depend on the needs of the individual footprint, so there is no right answer.  You need to reach a compromise between what the component manufacturer suggests and what the board assembly house recommend, given their exact knowledge of their manufacturing tools and process and the thickness of the past mask used.

1:  In Altium Designer a negative paste mask expansion means the hole in the paste mask is smaller than the pad dimensions.  I usualy start with zero.

2a:  To little paste causes weak joints.

and

2b:  Too much paste escapes across the board causing solder balls which short things out. Or can cause toombstoning or floating where the surface tension forces are such that they lift one end or both ends the component up during reflow.

3: Listen to the assembly house, they know best.

I should add, that the assembly house often alter supplied paste mask data to match their process as a matter of course when the setup the job.  Find out if they have done this and put the alterations back into your design, that way if you move houses or make a 2nd batch you can be sure of using the best data.