And if you have been riding with slightly damaged square taper cranks for some time it is possible the bottom bracket spindle is also slightly rounded out and any new crank you install might round out in short order. When you take off the crank check to see if the spindle looks particularly 'polished' on the edges of the flats - if it does then a new bottom bracket (~$25 for a good one) will save you having to get a third set of cranks in a few weeks. If you change the cranks it is likely that you will need to change the spindle length anyway to keep the chainline straight.

After you get everything replaced then blue loctite on the bolt threads will not hurt, although i just use a healthy amount of thick grease.

The square taper of the cranks and spindle, though, should have NO grease at all... and I have found red loctite applied on all the taper flats (do not put it on the bolt threads) can keep a creaky set of cranks in trouble-free service for years.