It seems like no matter what you do the pressure between the tanks is never perfectly even, and they rarely feed at exactly the same rate when you have both tanks selected. FWIW, I use a single vent under the wing to each tank, with a single pick up at the back in each tank feeding a small header tank located behind the baggage bay. Both tanks feed all the time and there is no crossover vent. My tanks always feed a bit unevenly, but they feed as evenly (or more evenly) as compared to the average Cessna that does have a crossover. The pressure differential is so low as compared to the weight of the fuel, that it really doesn't make any difference. Both tanks will draw down to as low as I am willing to fly them. Since I can get 100% of the fuel without loosing fuel flow, and the differential in feed rate from the tanks is low enough that I can't feel it and it doesn't unbalance the plane, I guess it doesn't really matter for me.
If you add a crossover vent, it will typically have to dip down below the top of the tank level to cross over to the other tank. So, the first time you top off a tank, that crossover vent will fill with fuel that will remain in the low spot of the line. With both tanks vented to the outside world, you will never see enough pressure to push the fuel up hill out of the crossover line to clear the crossover vent line.
-Cub Builder