I suppose it depends on the business but at my Fortune 50 we're required to use "lowest logical airfare". You can force a choice of Southwest over United by choosing one airport over another, and there's enough wiggle room to let you fly at your preferred times but overall it's designed to prevent gaming and escalates things for review if you try.
But unless you're a highly placed executive no one travels on the company dime often enough to bother with air miles anyway; airlines require far too much travel for even the bottom-most loyalty tier.