Via-in-pad is best done with small vias (< 0.3 mm ID), which don't wick solder very much (particularly lead-free, which flows more slowly than leaded).
Soldermask tenting isn't recommended because of the added height under the component (the thermal pad will rest on top of the soldermask), and the significant reduction in contact area (you've basically made a via-tent sized void in the solder joint).
Alternately, if you have a wave soldering step, you can possibly recover the pad solder by using large vias (>= 0.5 mm) and leaving them fully open. Wave solder wicks up and fills the vias and pad area. Or traps bubbles in the vias, because they're not open and vented at that point. Whatever.
If you can afford a custom PCB fab, filled and capped vias are wonderful. They are plugged with epoxy, leveled, and plated over with copper. The resulting surface is smooth and planar, without voids (maybe a perceptible dimple where the filler is, that's all).
Tim