When is the best time of day to top my plant?

I'm not sure what sex it is yet, its on its 7th node, I dont want it to get tall, I'd rather have a shorter bushier plant. And how its properly done, I would greatly appreciate it.

Any time is fine.

7th node? I do mine in the 3rd to 4th node (then again, I shoot for four colas). If you have transplanted recently, I would give them a few days to settle in. If they have been in their pots a while and are going to stay, like researchkitty said, anytime really.
If your THAT concerned, then I would top them just before lights out as that is when your plant does its real growing (hence, it can do some healing too and start those new cola stems).
Just my two cents. My current grow has plants that have not been topped. Some with two cola's, couple with four colas. Even have a mutant that is three colas and one of those colas is splitting on its own!!

Thank you!!

I have been topping and topping a great white shark for a month now

I top at the end of the day, nutes and water have coursed back down to the root system, seems to shock it a little less since the tip you're removing isn't saturated. The bigger the plant is, the less it seems to care.