Higher, in my experience, is always better.
The hotter a fuel/air mixture gets, the more it expands. There is a limit. If it gets too hot, the pressure in the cylinder reaches a point where the fuel begins to detonate. The cause of detonation is excessive heat and/or pressure. When the fuel starts detonating, the exhaust gas temperature goes down. When this happens, it is not immediately disastrous. If you see the EGT go down, just open the high-speed needle a little, or in the case of a fixed jet, head for the pits and put a little bigger jet in it.
Be aware that the response time of the EGT sensor is very quick. Coming out of the corner, if you see the EGT reading lower than previous observations at the same point on the track, open the low-speed needle. Once you get used to the intricacies of tuning with the EGT, I think you’ll agree with me that tuning with the CHT is just about useless.
“Tuning is tough” (Al Nunley)
“If your data does not support the theory, get a new theory” (Al Nunley)