I just did some initial testing (I’m writing up a post about it now), and while your assumption makes sense, it seems that it is not correct.
I just tested both MB and TBB on coveryourtracks.eff.org (a browser fingerprinting test).
In both cases. Moving from ‘standard mode’ to ‘safer mode’ made you (somewhat) more unique. But moving from 'standard mode to ‘safest mode’ made you substantially less unique (in fact the best configuration of any that I tested was MB in ‘safest’ mode, with TBB in ‘safest’ mode coming in second).
This is most likely due to scripts being blocked in ‘safest’ mode, so while it is true that going with a non-default configuration makes you more unique, blocking scripts makes up for that and much more by blocking the server from accessing a lot of fingerprintable datapoints. (so essentially “600 steps forward, 2 steps back”)
Another thing that surprised me is that you are slightly more identifiable if you keep the browser window in its default / unmaximized state. Maximizing the browser window had a slightly positive effect in ‘standard mode’ and no effect in ‘safest mode’ (because blocking javascript also blocks sites from being able to see your browser window size apparently).
So TL;DR at least based on my unscientific testing, using “safest” mode makes you less unique than “standard” mode for both MB and TBB even though you must break from the herd in order to enable this mode.