I'll say it again - I'm not a pro player in any way.
Just in my small experiance - she often is forced to farm safely and don't get pushed/forced to recall. For that I wouldn't max her W. I think - the less points I give there the more points I have on my more vital spells. I tend to start from pool and "max" it to the point it's killing caster minions in one go. Even on rank 5 it doesn't take the melee ones in one shot (if I remember correctly... may be wrong).
I tend to max first my Q spell - it's my initator, it's my "escape ability", it's my bread and butter. If I (by any means) screw my Q - I'm going to get harrased the hell out of me or in worse scenarios get killed. I usually won't get the kill if my Q is screwed anyway.
My E is my "protection" spell for me and for others. "Maxing it" gives me lower cd which is good because we want the cc reduction/immunity - not the power of the shield. Against Talon and the ones that "are thinking it gives me problems" the power of the shield is not the main benefit. It will last as long as you need for the cc reduction - you just have to play smarter and predict when he wants to jump on you with his silence and slows. Against bursty casters you have to play smart too - the dmg reduction is nice but it usually ends in one blow. That's why shorter cd on this is good. I tend to pick one point in it or try to "max it" after my Q. Depends on the situation.
Her R doesn't need explanations...
What I try to say is - if I go with Morgana for the kill on mid. It's usually the combo of Binding, Ultimate and Soil (just the skills, it's not a rotation) - Usually people can't survive this combo - whenever I have my W on 5 lv or have it on 2-3.
And in late game her dmg is not a thing to be scared. The "smart teams" will not let the ad carry/support get binded and wiped to oblivion. The bruisers and tanks will go "in front" trying to disturpt or "shield" the Q.
Long story short
It is vital for her Tormented Soil to cost as low on mana as it's possible and kill as many minions as possible. If she goes for the kill she "pulls" everything perfectly and gets the kill anyway or screw her Q/R and not getting the kill anyway. Conserving both mana and health (passive, lower costs and maybe blue (which can be given to someone else with proper setup)) and staying in the lane is the main point in laning Morgana. It doesn't take in account her utility and other benefits.