I've had a couple of juxtaposed experiences lately that have prompted this thread. I took GUE Cave 2, and squeaked by by the skin of my teeth with a provisional, and after I came home, I had an opportunity to dive with Bob Sherwood, one of the GUE Training Council guys, and get some feedback from him.
When I dove with Bob, we spent over an HOUR taking my rig apart and resettling it, resulting in dropping the tanks almost three inches. We reduced weight and moved weight, and some of that worked and some of it didn't. And we did very simple things in the water -- a valve drill, a little swimming, and clipping on a deco bottle, deploying and stowing the reg.
What I came home with is this: When I did Fundies, I railed against the strictness of the standards (although I tried to meet them, and eventually did). In Rec Triox, I got even more bitter -- Who CARES if you are 15 degrees out of trim during an airshare in midwater?
Well, what Cave 2 taught me is that all those things matter at some point. In high flow, being at all out of trim means presenting a larger surface area to the flow, and thus higher resistance. Going out of trim when you lose a fin means blowing the viz, and converting a minor issue to a major one. Taking several minutes to clip in a second bottle delays the entire team in starting their decompression, and adds to the O2 burden they are carrying.
The standards for Fundies, for someone doing Fundies level dives, can seem ridiculous; but the time to learn good habits (and avoid bad ones) is at the beginning, because it is far more expensive and difficult (and painful) to correct things at a higher level, later on.
Begin with the end in mind. Put the work in when you start, and you won't have to do it in spades when it really counts.