I respect Steve's opinion but mine is the complete opposite, what do you expect since I suggested it.
If you are a guy that has competed several times and knows your body very well there is no point to doing the trial run. If you are fairly new to competition, haven't peaked in a while, want to try something different then I think this is a good opportunity.
I plan it into my contest prep, I know I won't lose fat this week and the last week since when carbing up over the course of the week I will be at about maintenance calories. So I start dieting 2 weeks earlier to have time for it. If someone is late in their prep then they should skip it because being lean enough is more important than peaking properly. Just show up dry and not as full in that case.
I do exactly what I would do on show day including dropping water. I make sure to do lots of posing and take notes on when I look best, this helps me decide when to drop water, how much carbs should eat, etc..
I do think that competitors using prescription diuretics might screw themselves up because many guys over do it, this will affect the kidneys and there will be lots of rebound. It would be a mistake to use diuretics twice in such a short period.
If you are not using diuretics then the rebound really isn't that bad, the water is gone in 2-3 days. Remember you are going right back on the diet and besides it's just water even if you are an extreme case it won't stay on for more then a week once you are back on your hypocaloric diet and doing cardio.
Also, from year to year my body changes so I need to adjust timing and carbs every year.
If you aren't doing a trial run then I recommend being conservative with your carb load, the fact is, it's better to show up dry and flat then full and watery. Many people over do the carbs and look worse after the carb up in my opinion.
That's my 2 cents.