As internationalfish says, there's no harm in prepping yourself and your tools for the jam. If you consider that a lot of people won't have to. For example; people using Unity, GameMaker, etc have very little prep as all the tools are integrated, compared to those not using IDEs who have to sort out all sorts of dependencies, libraries, build systems, etc. Things like that shouldn't have to be done to any specific jam time, if they was you'd pretty much end up doing the same thing again for every jam.
As for starting early, you could do that even for a themed jam. But at the end of the day, there's no prize, only the fun and challenge you have while doing it and the sense of achievement at the end. Yes there's some friendly competition, but there's nothing to "win", so if you "cheat", well that's just on your conscience because no one else is going to know and/or care accept you. :)