It would give good value to do the inevitable all at the same time.
Other than economics there is no reason not to do just the one head. There is an allowable amount that can be cut from a single head and not screw the timing or fit of manifolds so obviously the slight squwish difference between new and old gaskets is negligable.
The only problem with letting it leak is where the oil is going. Its not nice what it will eventually do to exhaust or ruber products. The mounts and suspention rubber bushings can be decimated by long term exposure to oil.
Bosch Master, ASE Master, L1
33 years MB technician