Flange is more for slip down prevention. It's just a precautionary measure, not completely necessary. The only way the flange helps with "leaking" would be if the tank slipped down too much, but it doesn't help seal it any better.
If you're using it on an ego, you can buy this....
eGo Chrome Cone for the 5 mL Dual Coil Tank Cartromizer
It would give the tank something to rest on, preventing it from slipping down, similar to what the flange is doing. It would work on the 3.5ml tanks too, it would just be a bigger around the tank base.
Heck, even this would work for all size tanks....
eGo Chrome Cone for the 3.5 mL Dual Coil Tank Cartromizer
