To charge Hobbs time is probably a rip-off, and leads to pilot frustration.
What you will find is that in the long run, an aircraft owner/operator will simply tally up all costs for fuel, maintenance, hangarage, insurance and everything, and divide that amount by whatever amount the hobbs/tacho/whatever racked up in that period. That, plus an uplift for the owner, is your rental fee per hobbs/tacho hour.
So it's true that on a typical one-hour flight your hobbs will show 1.1. or 1.2 while the tacho shows 1.0. But at the same time, all else being equal, the price per hobbs hour will be 10 to 20% lower than the price based on tacho hours.
Unless the owner is using your ignorance to rip you off of course.
The reality is that there is no easy, tamperproof and accurate way to charge fair rental prices. After all, insurance and hangarage are typically a fixed amount annually, tires only wear out from landings, taxiing and take-offs but not from airtime, fuel is both used to taxi and to fly, 50-hour checks are based on airtime only and depreciation is outside your control whatsoever. So to do this properly your rental fee should include the miles you taxi, the airtime you have, the power/leaning setting you used while flying, the amount of landings you made and a few other factors. Simply not doable. So most owners go for tamperproof, easy and more-or-less-accurate.
At my club, most rental fees are based on tacho. But the Seminole twin and the Thielert-equipped planes (DA40 and DR400) are based on hobbs. In our rental fee overview the last ones are clearly marked as such, and the hourly hobbs fee, all else being equal, is 10% lower than the hourly tacho fee. Precisely because the hobbs, in a typical flight, will show 10% more than the tacho would.