That is one way to do it; now you know the proper number of vias are placed. Others will want to leave the vias out and place them during layout and routing. For instance if the vias at pins 15-17 had to be moved for a critical trace to route on an inner layer, then three other vias could be added at the 21-23 locations.
I like your way, as you can always modify the decal locally if you need to delete unused pins (vias). When you 'update from library' you'll have clearnace violations until you repeat the modification again, but that is what DRC is for. Another plus for this technique is that you have better control over the solder paste definition so you aren't pasting the vias.
There are pros and cons with every technique.