Magician: Essence=6.
Augmented: Essence= as close to zero while still staying above that I can get it to come out.
Adept:: Probably 5, but have number crunched number of a essence 4 adept.
Personally, I won't make an adept with more than 4 essence. Too many of the adept powers are just better handled with bioware, and some augmentation effects can't be replicated by adept powers. Some cyber is good, of course, but I limit it to low rating cyber ears (mostly for dampening and balance augmenters), data jacks, and
maybe cyber eyes (but usually not). Either that, or a cyber forearm and a cyber full arm are nice touches if you're making a longarms or assault rifle adept. Or just a cyber arm if you're making a pistolier.
I just crunched a great augmented adept who is a face/hacker/wall-running infiltrator (sounds stretched thin, right? You might be surprised how well this works when you do an augmented adept!). For an adept, I wouldn't go below 4 essence under any circumstances. Lower than that, and you have to make real sacrifices with adept powers, then you also start spending more on wasted magic points so the costs no longer justify going that route.
For a samurai, on the other hand, I see no point in being stingy with burning up essence. It's like not filling up your gas tank all the way because you might want to put more in it later... Sure, it's not
actually that simple, but TO HELL with magical healing!
For a juicer, I might try to hold on to 3, just so I don't die from my primary stat-boosters.