I think that's a great point! A lot of times, if it's too difficult to get the bottom, that may not be the bottom you want to be fishing (for Steelhead anyways). Here is a pretty interesting study done you may want to check out. It's definitely an enumerative result, but pretty cool none the less.

http://flyfishingresearch.net/flydepthrules/flydepthresults.html

I don't believe anything in fly fishing can be taken as 100% accurate all the time. There are too many variables to consider, but the information given here is worth taking time to consider.