I have just as many S&W's with the lock as without... it is no big deal, believe me. I have never had a problem, other than wondering what that spot I couldn't wipe off was from some four years back (My first 4" 625-8 .45 ACP). I now use my reading glasses when I clean my guns. While I wouldn't buy one with the lock if I didn't have to, S&W is making some fine firearms now, like your 620 (It's on my list...), to say no to the lock keeps one from experiencing some fine new revolvers... not me!
I'd unload it and remove that grip and make sure that strain screw, on the lower front of the grip frame, is tight (CW). Then, I'd flood-spritz a bunch of a good aerosol cleaner/lube, like RemOil, into the innards, wrap the grip frame area with a paper towel and shake to remove the excess - repeat if much metal residue, etc, is present. Shake it fairly dry and re-install the grip. Lock your ammo away somewher else, find a garbage TV show and dry-fire the snot out of it - changing grip hand every 100 shots or so - to at least a thousand times. Then, repeat that first step and leave it with little cleaner/lube. You'll have a slickened trigger - possibly some lighter in DA, as well - at least your trigger finger(s) will be excercised!
Get a HKS #587 speedloader - they fit the 686P's 7-shot cylinder. Actually, a 620 is simply a 686P cylinder and frame, with a last 4" 66's two piece barrel. I don't care for the open back Hogue grips, so I would either put on an Uncle Mike's Combat grip, probably squared like my 66 came with. The rounded ones feel great, too - and they both cover that backstrap. Of course, any K or L frame round butt grip will work - grips/stocks are highly subjective. Congratulations on a fine purchase!
Stainz