I've used C41 and E6 plenty. As the offerings dwindle, I've got some Bellini C41 teed up next and paused over the lack of mixing instructions relative to Temp. Arista's pretty clear here that you mix at 102/104 and so I'm inclined to do that. Seems pretty much the case for many B&W formulas as well. Am I missing something? To wit: "When in doubt, heat the juice to 104 F..... or so." Not too hot and not too cold.
The constituents of a typical C41 chemistry set are all easily soluble especially at working strength concentration. Anything between 'room temperature' and 50C or so will be fine. The higher the temperature, the quicker you're done. If you're starting with liquid concentrates instead of powders, it doesn't matter at all since the process is completed more or less instantaneously anyway.
For all developers, stop bath and fixer, color and black & white film and paper developers, I follow the manufacturer's temperature ranges. The ensures that the mixing will be thorough and there will be no absorbed chemical particles.
Sometimes when chemicals are mixed, small particles remain that do not go into solution. There are many threads here that people want to know if they should filter out those particles, heat up the solution, stir the solution longer, ... That is what I am referring to. Since I started carefully monitoring the solution temperature as I mix them, I have had very problems with absorbed particles.
FWIW, Fuji recommends mixing their c41 developer replenisher somewhere in the 90-100f range. I've always just mixed at room temp and have never had any issues.
If all C-41 chemistry is the same as they say, then use Kodak manuals. Z-131 is the bible of C41. It says that the mixing temperature can be anywhere between 70F and 100F. Page 1-10.