Shims for the diffs. All I can say is I ran the truck for 4 weeks on 6S with the stock, unshimmed diffs. During that time I had some nasty crashes, including one that sheared the screw off that holds one of the front shocks. I also bent the rear shock tower a bit. In any event, my diffs were fine. I ended up shimming them because I like to tinker and the shims are dirt cheap, but I am not completely sold I HAD to do it. And on top of that, I couldn't even fit the recommended number of shims in without the differential being very hard to turn. I could feel the gears grinding against each other. But this is the bottom line: I always replace the bearings on RTR's (I think Arrma's stock bearing are pretty poopy.) So if you are going to replace the bearing you are going to have to open the diffs. And you are going to have to replace the diff lube when you do that. So 1) change the lube weight because the stock weight of 10k/10k/10k is way to light, 2) Dry fit some shims and see how it feels and 3) replace those bearings. So while I don't think shimming is an absolute must with the V3 diffs, you are right there anyway so see if it needs it.
My last diff swap - I bought a full V3 diff from Ebay. The V3 diffs are much tighter than the older diffs - and this is good. BUT - I still put "shim the diffs" on my must do, - not because I think you need to add shims, but just because this is something you should check. Every diff is different. With the V1/V2, most of the diffs need more shims, with the V3 most of the diffs don't need more shims - but every diff should be checked...
My .02