Large vs small stack & reach
I think what you are getting at in your question is: is there and optimum bike geometry for any given rider and riding position or style? Yes, there is, but stack and reach are not the measurements used to specify bike fit per se and they are not the only frame geometry parameters that affect bike fit. Seat tube angle is also important as it affects where the seat is located.

The basic measurements used in bike fit deal with the three contact points on the bike: feet, butt and hands:


Saddle height above bottom bracket
Saddle horizontal set back aft of bottom bracket
Horizontal saddle to bars distance
Saddle to bars vertical drop


If you know what those are for a rider you can pick a frame with stack, reach and seat tube angle that will allow seat, seatpost, stem and handlebars to be positioned and adjusted to get that fit. 

Of course, that is not the whole story. The steering geometry that is influenced by wheelbase, chain-stay length, head-tube angle and fork trail are big factors in bike selection as well as cockpit geometry.   

I think because stack and reach, along with other frame geometry parameters are published by bike manufacturers people thing they need to find 'their' preferred or optimum stack and reach. I would not pick a bike that way. The best advice is to test ride an array of bikes at a number of different stores and let bike store staff give you some advice on sizing and fit. See what feels best for you.