If you want your diffs to last longer then i'd reccomend it. All you really need is a file, a grinding wheel just makes things easier. I'd do the mod, but can't find anywhere that actually has the correct size washers.
Today i took my stock MT-A4 (.21) here....
http://www.nnrc.co.uk/
Removed the shock spacers to drop the truck and borrowed some half decent pro-line tyres (can't remember exactly what they're called) and the truck was amazing. Handled like a dream, the engine was running perfcetly (one other racer thought it was the .28 it was that rapid). Even pulled a few wheelies. Beat the body up a bit trying (and occasionally succeeding) to clear the table-top. Destroyed the body mounts, but still managed to keep the body on the truck. Snapped a hub when i ran head-on into a HPI Rush at speed, the guy that owned the Rush also had an MGT so he gave me a free hub! Good guy (considering i broke his Rush...) And one of the screws in the drietrain kept coming loose, despite being threadlocked in place! Another racer had spares of those also, and was kind enough to donate four of them.
Went through nearly 1/3 gallon of fuel and only left the track when i ran out of fuel. Great truck!