When to repitch yeast

With some high gravity recipes, it is suggested to repitch more yeast toward the end of the fermentation to make sure it is fully fermented. At what gravity should I consider keeping some yeast to repitch and when would you do it?

How high gravity are you going for? As long as primary fermentaion was moving along, it isn't necessary. A hydrometer is a big help. If you do something like a barleywine at 10% abv, the OG is going to start somewhere at 1.100. The final gravity will only get down to maybe 1.025. If it stalls somewhere around 1.035-1.040 or even higher, more yeast can be help. Make a good yeast starter, or even better, pitch on top of the entire yeast from a previous batch and you'll most likely avoid this problem.

I really wasn't shooting for anything more than an OG of about 1.070-1.075. So you're saying I should be ok, but just make sure I've got some yeast if the hydrometer says I need it?

That's about it. I like to use the hydrometer all through the brewing process just because I'm a nerd. Others around here will say to throw the hydrometer away. Checking to see if the primary fermentation is done is one time where I think it's really necessary.
Just make a big starter with the yeast and save some in a bottle and keep it in the fridge. If you don't need it, you now have another yeast sample for another brew.

If the attenuation is around 60% after a week, I like to add some distiller's yeast. For stouts, I'll use champagne yeast. I never use either for the primary yeast, just finishing.