I can report from experience that the carb on an O-540, as used in the PA28-235, holds enough gas for a three minute ground run, a normal runup, and the beginning of a backtrack, having been cleared for take-off. Halfway through the backtrack the engine died. The fuel selector was off.
I would not be inclined to trust that the time spent on a run up guarantees the tank selected is providing viable fuel, unless this can be ascertained by experimenting with actual fuel duration by testing this procedure with the selector in the off position.
It is probably safe. Carbs are of differing sizes, in different engines. Obviously the float chamber in an 0-540 is likely to be a fair bit bigger than that in an 0-320 or 360. But then, the fuel draw will be a bit higher in the larger engine, too.
Test it. Don't rely on an unproven figure. I would not be inclined to change tanks prior to runup/departure just to ensure that they were all feeding correctly, unless that test was comprehensive, and indicated.