- ROOT - WHEN, WHAT, AFTER

WHEN TO ROOT
Depending on the method used, ROOTING imposes some convenience constraints.
a. If you are using the first method to ROOT, you can do it at anytime since the existing system is left untouched - your apps, customisations, settings, data are all there.
b. If you are using the second method, since the entire system will be wiped, you should ideally do it before before you have loaded your phone with a lot of apps and have customised it just the way you like it. This is not just about taking a backup of your personal data; your apps, settings and customisations cannot be backed up and you will need to redo all this in the new system.

WHAT TO ROOT
If you are going for the method 2 ROOT process, you should be careful about which ROM you use. ROMs are tailor made for a device taking into account it's hardware features, so the ROM of  a phone with one camera will not have the routines to operate the second camera in a phone with that feature. That is, ofcourse, a sunny scenario. I am sure using the ROM of a phone with a dual core CPU on a weaker one will catastrophically overwork the latter's CPU.
While many posters at forum.xda-developers.com have been adventurous with flashing ROMS of different phones, without mention of any fatal results, it is best to seek advice and blessings of the elders there before it is too late.

The method 1 ROOT process is safer since it doesn't fiddle with the Android system on a large scale.

AFTER ROOTING
Unless you are planning to use the ROOT permissions frequently, it is safest to unROOT the phone once you are done with it. There is always the possibility of unwanted apps entering the phone and then using the ROOT permissions to do greater damage than would be possible without it. Why talk about nefarious apps? Even your 5 year old nephew could inadvertently press a few buttons while playing Angry Birds and leave you with a dud.