I agree with the others that you need to understand the cause of the dendrites -- although since your boards haven't had this problem, that will be difficult.  Prove that yours won't do this?  With clean boards and processes, you should not get dendrites without specific circumstances mentioned by the others.

As for issues with tenting vias, the solder mask down in the hole will not cure fully.  When it goes through the SMT processes, it can volcano and spew solder mask gunk on the boards.  We used to do tenting, but had problems and were advised against it by our PCB fabricator.  We now use "zero" hole clearance on the mask, with the opening just a few mils larger than the hole on both sides.  This still exposes copper, so may not satisfy your customer.  But it leaves a clean hole and no spewing.