Personally, I wouldn't worry about whether the ammo is sealed or not. As long as it's stored well, it's really not going to matter all that much. Now if you are a Navy SEAL who will be doing dives with your weapon and ammo, then you might need to be more concerned. Otherwise it's not that big of a deal.
My method of storing ammo is to pack it away in ammo cans in good condition on a day when the humidity is low. Of course I also have a dehumidifier that can make my ammo/weapon room as arid as Arizona in a jiffy. Pack the ammo away (leaving it in the boxes), drop in a dessicant pack and then seal the can...leaving it in that condition until needed. I also label each can as to caliber, load and date for easy future reference. Place the cans in a cool, dry location and you are done. Avoid putting them in areas where the temperature gets too high. A temperature of 70 degrees would be good. Temps that climb above 80 degrees or warmer may not be the best though. Heat will deteriorate ammo faster than anything.
I also keep about 250 rounds on strippers but don't feel any need for more than that. My attitude is such that by the time I run through 7-30 rd mags (210 rds) and the extra 250 rounds at the ready on strippers, I will either have long defeated the threat or been killed. Any one individual is not going to last long enough in a gun battle to use more rounds than that anyway.
-CH