says to switch to one tank or the other for cruise.
The Cessna flight manuals I have reviewed use phraseology like "The use of left or right is reserved for level flight". That is not an instruction to
use one tank or the other in level flight, but rather an instruction to
not use left or right during maneuvering, takeoff, climb, approach or landing.
If, as the pilot, you feel that you would like to confirm the operation of the fuel system independently, you are free to do so. However, this is not a routine check. To be effective, you're going to have to run the engine for several minutes on each tank before switching, and very certainly more than that minimum number of minutes on "both" before takeoff. It takes a while to run a fuel line dry! If you suspect that the fuel system requires an operational check, than of course you'll also turn the fuel off to assure that the engine stops - right! If you are so uncertain of the proper operation of the fuel system, that you feel you need to check fuel selector operation, and flow from each tank, you probably should refer the aircraft to maintenance staff.
From a safety standpoint, you risk more accidentally taking off on only one tank (or fuel turned off), because you forgot to finish your check, than you're ever likely to turn up doing the check. If Cessna felt this an important check, they would have included it in their checklist, and I have never seen a "change tanks" check in a pilot's preflight checklist. Flying schools who introduce such checks as preflight, in their own home grown checklists are doing a dis-service to pilots by causing them to think these are routinly needed. They are not routinely required in airworthy aircraft (or Cessna would have specified them!).