Prewiring experimental racks
I think that you should prewire experimental racks for power. By this I don't mean just making sure that they have some PDUs in them; I mean running actual power cords up the sides, with unconnected ends tied off every so often ready to be plugged into machines. If you turn out to need more power cords for a machine, don't try to run more power cables along the side unless they too are going to be permanent; just dangle them down the back, where they are easy to remove later.
The goal is to keep things neat, instead of the situation you get from pulling, repulling, pulling out, and so on a snarl of power cords as you move test machines in and out of the rack. Unused but organized power cables may not be ideal, but they are a lot better than the real alternative.
The power cords should have enough slack to reach to the other side of their rack slot, since you never know which side of a machine you're going to need to plug them in to. If you have multiple PDUs in the rack, label the cords with what PDU they're plugged in to; if you have smart PDUs that can power cycle outlets, label both ends with what outlet number the cord is in (or at least should be in).
You may want to do this with network cabling too, depending on the cabling density. This is one case where a smart switch at the top of the rack may be a lot less messy than the alternatives, and you're unlikely to need lots of bandwidth out of your experimental rack unless something very peculiar is going on.
(This thought was prompted by some machine shuffling I did in our experimental rack, which has something of a power cord snarl problem.)