As with most things brewing, the best practice is what's best for you. The idea is to have healthy, active yeast in the quantity necessary to start fermenting your wort as soon as possible and to be able to finish strong.
For me, I like to start my yeast so that I am pitching into the fermentor while the starter is at high krausen. The yeast are happy and healthy and ready to work. The time frame will depend on how old your yeast is and how much you are trying to build it up. 12-24 hours before pitch time is usually what I shoot for.
Yeast is a very deep subject, if you want it to be. There is a lot to read about, and a few different schools of thought. Just keep brewing and experimenting and once you find what works for you keep doing it.