The tail gearbox itself isn't "level" until you bring down the boom a bit, therefore you lower the boom slightly, which you have to do anyway to check the aft flex coupling.
The other thing one can do when checking the tail sight glass and aft flex coupling, is you can do a check of the main blades when each comes around to you, the angle gives you a good look at the upper surface, etc.
I also look at the tail blade drain holes at this point, as since I'm holding the boom down, I have a good view down to them, rather than looking up as they spin by, (also noting that the blade tip weights are installed)
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Marc