Depends on your height and situation at the time but I'd generally agree with Beagle; you'd only get a couple of hundred feet at best by converting speed to height and if you're already at 3000' then it's not worth it. If however you're at ~1000' or below then it could mean the difference between making your field or not, especially if you're fast at the time and therefore have spare kinetic energy to convert into potential, which is what it comes down to.