I used to follow the upgrade path on my bikes, but nowadays, and especially since I found Retrobike I prefer to keep things period correct and more or less factory spec.
That said I haven't bought a complete bike since 1992, always preferring to buy a frame and build up to my own spec.
But, for example, I wouldn't build up a mid range frame with XTR and boutique components. Nor would I slap 200GS on a high end piece of titanium exotica. (if I had any)
I like building up bikes around groupsets though.
I currently have appropriate bikes built up with 200GS, Deore LX M550, DX M650, XT M732/5, XT M739, XT M750, STX, STX-RC etc. I'm also working on 400LX, Deore II, LX M563, LX 565/7 builds and a few other various bikes.
Hubs and rims have to be period correct, although I also seem to have duplicates built with modern rims too for winter riding.
There is clearly a touch of OCD in all this, but I justify it by riding them all and getting to compare component performance and bikes.
This approach has spilt over into my modern bikes too with XT, X-9 and Deore based bikes.
I've also tended to upgrade to XT level and then stop. I've had XTR stuff, but generally can't justify the cost to performance ratio, so XT is perfectly adequate.
I guess I'm ultimately working towards a bike for each Shimano groupset from the late eighties and nineties.
God help me when I move onto Suntour, Campagnolo, Sachs and Mavic, or indeed road stuff.