:dowhat
You want a ratio that gives you the best spread and takes the longest time to spool up the drum. Nothing wrong with a 6th gear run, it's just harder to see what's happening down low as it's hard to get the engine to pull cleanly from near idle at WOT. 3rd can be ok, it will pull from down low ok but the run will be over in a shorter period of time and you have less samples to create the graph. 1st & 2nd, traction problems, run time too short and internal inertia starts to skew the results too.
(Nothing is close to 1:1 You've got primary drive ratio, gearbox ration and final drive ratio. If you select a gear that's closest to 1:1 the primary and final drive ratios and wheel size and tyre profile all affect the engine to drum ratio.
And, 6th gear is the closest to 1:1 in the 03/04 and the 05 ZX6 manuals I just looked at 1.153:1 (30/26), the TL is exactly 1:1 in 6th (24/24) my GSXR1000 is closest in 6th too with 1.208:1 (29/24) So whoever told you that, go call them out, if they were a dyno operator give them a :bitchslap for havng no idea what they're talking about.)