In the PA32 Cherokee six, you have 4 tanks (2 x 17 usg, 2 x 25 usg) with a total of 84 usg. You can select each tank in turn, but there is no facility to select "ALL".
To get maximum endurance safely, the POH advised running some of the tanks dry, in order to avoid the situation where you arrived at a destination with sufficient fuel in the tanks for reserves, but insufficient in any one tank to reliably cover the fuel outlet.
The advice, which seemed counterintuitive, was to run the two inboard tanks dry first, and then run on the outboard tanks; the reason given was to unload the main spar.
IME, the process was a non-event just so long as
a. you were prepared for it and switched tanks smartly (remembering to disengage the altitude hold on the autopilot)
and
b. You had sufficient altitude to deal with the exercise safely. Typically,the time between the engine starting to cough and full power being restored was 7-10 seconds.
Quite important to let any passengers know what is going on too, lest a 'brown-trouser' moment result. I only forgot once, but had quite a memorable response.....