Why do you want an adjustable regulator? The required charging voltage varies with temperature anyway, more than the accuracy of a fixed regulator. It makes as much difference as using gold plated battery terminals (the connection weakness is actually the cable to post rather than the post to battery). But that is only my 2c as an electronic engineer.

That said, if you do a lot of long distance driving (a number of hours per day) then it is a good idea to run at a slightly lower charging voltage because then you are doing trickle charging instead of boost charging. Normally the alternator is designed for boost charging because usually you do not drive more than 1 or 2 hours per day.