I tried the 'if it ain't broke don't fix it' approach with one of mine and it broke anyway
Personally I am sceptical of the dislodged crud theory as all the auto boxes I've opened up have been spotless inside. I suspect it is more to do with running the box low on oil on startup, which is inevitable due to the stupid filling procedure. A gearbox which is just on the limit of working properly could be tipped over the edge by running short on oil for the minutes it takes to top it up. I have no solid evidence for either theory though. Just that all 5HPs will fail eventually whether you service them or not, and servicing may or may not shorten or extend their life.
However, as the guys have said, this is not something to be bodged or short-cut because if you get it wrong (even to the tune of a litre or so) you're pretty much guaranteed to break it. The converter is first to suffer from low fluid and it'll burn out its lockup clutch. Next is the oil pump, which will cavitate on hard acceleration or corners. Cavitation leads to metal fragments being circulated through the valve body and clutch cylinders and then its all over. If the pump survives but the clutch cylinders can't get full travel, and the clutch packs are already worn, then the clutches start slipping and burn out.
Side note - don't believe the Wheeler Dealers episode where they magically fixed a broken Porsche Boxster 5HP19 gearbox by doing an oil change! Not only did Edd show an incorrect filling procedure (the engine wasn't running because it messed up the audio), but no 5HP in recorded history has been fixed with fresh oil. I guarantee they put a replacement gearbox in that one and glossed over it because it meant Mike's imaginary profit went up in smoke
Do it properly or don't do it at all
Edit - I took too long writing that! Hard shifts can also be caused by the MAF failing. Check the engine load at idle fully up to temperature with accessories off. Should be no more than 16-17%. Above 20 means a bad MAF. If you're in the US then at least its easy enough to get the Transgo kit and possibly a rebuilt gearbox if it does go bang.