65% to 68.5% mitigation is an effective 5.3% increase. 68.5% to 72% mitigation is an effective 5.1% increase. Depreciation in this regard.
However, 65% to 68.5% mitigation yields an effective 11% change in damage reduction, and 68.5.% to 72% mitigation yields an effective 12.5% change in damage reduction. Appreciation in this regard.
Back on topic...OP, I would suggest changing your damage trinket to the on-use version (Badge of Conquest).
The agility bonus is higher than its Insignia counterpart, and being on a mere 1-minute cooldown it can be macro'd into your shadowdance for additional burst on demand. Using the insignia is not a deal-breaker though.
Garrote serves 3 purposes:
1) Acts as a non-dispellable DoT. Useful against rogues and ferals. Dwarves are an exception due to Stone Skin.
2) Silences casters for 3 seconds (4.5 if glyphed).
3) Applies your Sanguinary Vein buff. As Assassination, it applies an inferior version of Venomous Wounds (since garrote ticks every 3 seconds as opposed to Rupture's 2-second ticks.)
Unless you're ganking lowbies or undergeared players that'll die in no time, it is a safe bet to include garrote in your opening sequence if specced into Subterfuge.
If for whatever reason you decide to open with cheap shot on a mage and they blink out of it, a nice option to keep them in place is to use Shiv. Using paralytic poison causes shiv root the target for 4 seconds (provided you have at least one stack of it on the target). I've never had this root break on damage, so it's a pretty reliable means of keeping a mage (or any other player) still while avoiding use of a Kidney Shot that will suffer from diminishing returns.