My understanding is that HCG should be used from day one, for 3 reasons- 1. It is better to prevent any testicular shrinkage which most likely will happen at second week (although it will not be visible)
2. HCG should keep testosterone up (for example after first injection of test e natural production will be reduced/stopped and in first week or two, before test e kicks in there will be drop on tt levels, using HCG will prevent this, other option is using some other gear for "kick start" cycle- but even in that case HCG must be used as testicular shrinkage will be fast as well
3. HCG has positive effect on other hormones as well, and also it stimulates thyroid- and if thyroid works faster than higher testosterone levels can be reached

HCG should not be used as part of PCT, I've seen people using clomid and HCG together but this is big mistake and HPTA will not start working until HCG clears off.