-6 ftw! I remember when i first realized i didn't use this value and figured out i actually could aim pixel-perfect now, but at the same time the forward rocket jumps felt really, really awkward in the beginning as i aimed too far away from me on the ground i think (or perhaps it was the other way around).

I don't need to check my eyesight to know it's terrible (cataracts, no binocular vision)
I'm not saying I can't see the SNG mega with -6, just that it's easier with 0 when playing a game at full speed. I'm more likely to notice it in situations where I'm not specifically checking for it.

Most people I knew pre-MQWCL were playing with cl_bobcycle 0 which was the equivalent of v_viewheight 0 IIRC.
Viewheight 4 is just the most extreme height you can go to and some players choose that for various reasons - prefering the perspective, increased vision, less disturbance during shafting (especially for LGC).

Simple use both of it!

here a example for you:
alias +viewheigh "v_viewheight -7; fov 90"
alias -viewheigh "v_viewheight 4; fov 110"

bind mouse4 "+viewheigh"

greets.

Think switching viewheight depending on the situation has most potential indeed. I would do it opposite combined with an eventual zoom though, and use viewheight -6 instead of -7, like:

alias +viewheight "v_viewheight 4; fov 90"
alias -viewheight "v_viewheight -6; fov 110"

bind mouse4 "+viewheight"

since when you actually need to have a higher value of viewheight, you usually knows where your opponent is coming from and then its more reasonable to zoom as well.