There are three things to consider:
- Conduction loss. As noted, higher VGS(on) reduces this, albeit incrementally past a point, which also depends on ID with respect to overall device ratings.
- Gate drive loss. Normally a small part of the overall total, but as it's often localized to a small gate driver or controller IC, and maybe a resistor or two, dissipating a few watts here could be quite problematic.
- Switching loss. This is a more subtle effect, but consider this: the bulk of the switching process (commutation) occurs over a narrow band of gate voltage: the Miller plateau. The amount of current sourced/sunk to the gate during that phase, determines the duration of this feature. The plateau voltage also depends on ID and VGS(th) (increasing with both), and is further modified by source-terminal impedance (especially inductance, raising it during turn-on and lowering it during turn-off).
Fast switching requires a large voltage drop across the gate drive circuit resistances. This is easier achieved with higher VGS(th) and higher VGS(on). Therefore, this is a big concern when using "logic level" types (typically VGS(th) in the 0.8-2V range), for example.
This effect is directly observable in datasheets, when switching times are given. Turn-off is slower than turn-on because the plateau is generally below half VGS(on).
There are even cases where negative VGS(off) is desirable. This draws more current through the total gate resistance, speeding turn-off; it also provides insurance against dV/dt induced turn-on (i.e., a sudden change in VDS drives charge into CGD and CGS, changing VGS).
There are many transistor types, load conditions and optimization goals to choose from, so the most general advice I can offer is: try different values and see what works best.
Most likely, drive around 8-15V will suffice, and losses can be better controlled by adjusting gate resistance (including using a diode to get asymmetrical rise/fall), timing (if applicable), strays in the switching loop, and other components (like inductor and capacitor losses). Not to mention synchronous rectification.