Storing paint bottles upside down?

Don't know about the others, but Tamiya acrylics are among the longest lived of paints. I have some which are 20+ years old which have not required upside down storage and are quite ok.  

if your cap is on the bottle tight enough that paint won't seep out the threads when standing upside down,,,,,,,then they are tight enough to keep the air out if you store them upright

the seal on a model paint bottle is not the threads, anyway,,,,it is either the outside edge of the bottle up against the bottom of the flat cap liner,,,,,or the inside of the bottle into the sealing lip of a bottle (or cone liner on those types of caps

the Vallejo type of caps have the tip with the opening going up into a sealing area in the screw on cap

you get much more life extension just by keeping the threads in the cap and bottle clean,,,and wiping out the cap before you tighten it down

I certainly agree with TarnShip on keeping those threads clean. Once I was on the upside-down bottle kick. What you do is to waste paint that adheres to the lid and dries as you have it off and make a general mess as it will eventually end up on the lip and threads of the bottle (I primarily use Tamiya paint). I am now in the habbit of stirring the paint instead of shaking the bottle, then whipping the lip with a solvent (glass cleaner).