If departing an Aspen or an Eagle, just forget derate/flex thrust completely
Absolutely the case that some circumstances dictate the use of rated thrust. As a wise checkie once counselled me "Lad, read the Ops Manual with a modicum of commonsense"
Likewise I have never inclined to the view that one should make a non-limiting takeoff into one by, for instance, flexing to the limit. For that reason I always liked Wal Stack's approach when he introduced flex to Qantas - leave a pad for mum and the kids in the calculations .. that also made the new procedure a lot easier to market to flightcrews.
I have no inherent problem with running up the operating lever a bit. Only three caveats -
(a) know where you are with respect to Vmc
(b) do it ever so measured and steadily
(c) have some practice exposure to the exercise in the sim prior to chancing it in anger ....