Low boost is handy for wet greasy days - otherwise run high boost all the time. Running high boost doesn't mean a thing unless you are boosting. That is, if you are cruising at say 80km/h in 5th then you are not boosting at all so it will make no difference what your boost setting is. Shift bck to third and floor the accelerator and you will be boosting hard, you would generally want all your boost so leave it set at high. On wet greasy days have a low boost setting of say 10psi so that your boost comes on much gentler and doesn't peak hard, you will not spin the wheels as easily and you can drive with more control - whats the use of having high boost and watching all that power go to waste anyway.
BTW your fuel economy will not change much at all unless you are boosting all the time. When you are cruising as above, you are really driving an in line 2.5 litre six and will achieve a fuel economy that reflects this. Of cuase if you boost you will increase fuel consumption proportionally.