What are you figuring the Sq ft of? A lawn, patio, driveway, parking lot, house? And for what? Mowing, spraying, granular, brick pavers, plowing? I usually have a pretty good eye on the smaller lawns, but thats just for a fert application. Mowing estimation is just an acquired skill that I have to admit, am pretty good at.
I measure all properties for mowing,fert.,mulch etc.. It really helps in figuring $ amounts and qty. of material. Yes, I can look at a property and give a quote without the wheel..but why not know best price to charge for profit and not "guess". Some lawns are small and I know I'll charge my min. price anyway. I think walking the property with a wheel also gives you the opportunity to find problem spots and look more professional to property owner. Another contractor here calls it his "wheel of fortune".