Points to Ponder
The best thing to do is test and document performance for your specific airplane. However, baring that here are a couple of points to ponder for a stock (non flop tube) system built to plans:
BASELINE PRIOR TO TESTING
The fuel tank is designed with a ribs acting as baffles within the tank. This prevents sloshing to some extent and limits the impact of side-slip on fuel flow. However, no specific testing has been accomplished to determine sideslip effects on fuel flow, so it is recommended that as a general rule, uncoordinated flight or sideslip should be limited to 30 seconds in duration (or less) when the fuel level in the selected tank is ΒΌ full or less. Unusable fuel quantity may increase by an undetermined amount with the severity of sideslip. If a power interruption occurs during uncoordinated flight, the fuel pump should be immediately selected on (if not already on) and coordinated flight should be re-established.
The appropriate emergency procedure could be: 1. Coordinate flight; 2. Boost Pump - ON (if not already); 3. Fuel Selector - SWITCH TANKS (if desired).
Generally, for landing, gas should be selected to the fullest tank. Typically a slip correction to adjust altitude on base occurs to the "inside" of the turn, i.e., left turn, left slip (left aileron/right rudder), so you can think things through prior to beginning the base turn. As Pierre said, the optimal technique for slip to correct cross-wind doesn't occur until the final phase of the approach, so that shouldn't be a factor.
TEST CONSIDERATIONS
To test, you first need to determine your minimum fuel/reserve requirement. the FAR's specify 30 minutes for VFR and 45 minutes for IFR. Many folks use 1 hour. Generally, it's best if that reserve is in a single tank vs. split. Thus, fuel up the "test" tank with the desired quantity and takeoff/climb on the other tank. Switch to the "test" tank and fly the slip sequence at suitable altitude. You have two options: (1) you verify that the engine runs properly slipped to the "low" tank at minimum fuel; or (2) you continue to repeat the test at reduced fuel to determine at what point you will unport and cause starvation. If you choose the second option and do not have a fuel totalizer, you can switch back to the full tank after unporting occurs and measure the remaining fuel after landing. If your tanks are not the same configuration, i.e., one flop tube, one standard pick-up, then separate testing will be required for each side. You should only perform option (2) if you've already completed testing to ensure adequate restart after running a tank dry (this normally occurs during Phase I testing, but if there is any doubt, then you should complete this test prior). As Stephen said, the flop tube/trap door should offer an advantage (assuming sufficient fuel is in that tank).
AUTOROTATION EFFECTS
One other area to consider is spins. If you intentionally spin the airplane, unporting can be a factor with low fuel states in one (or both) wings. As a rule of thumb, the boost pump should be selected ON. Above normal reserves, unporting is unlikely, but if there is any doubt, the tank opposite the spin direction should be selected. Due to the rapid spin and positive recovery characteristics (aircraft loaded/tested/documented within Van's limits), if usable fuel remains in the unported tank, an "auto start" is likely as soon as the recovery phase is initiated, unless prop stoppage occurs. If not, it will be necessary to initiate an air start.
Fly safe,
Vac
