Nute mix questions

Im seeing a lot of posts where people put up their nute mix and I keep seeing a lot of people using veg and bloom nutes at the same time.

Is this what should be done?

I only ever use the veg in veg and bloom in bloom.

It all depends on the manufacture right? I just follow the instructions...for the 3 part GH you use all three in different amounts depending if you are in veg or bloom.