This is a zombie thread, but WTF. I have run a LOT of beer thru my swing-top Grolsch bottles over the years w/o a problem. I also have a lot of liter size swing tops my dad gave me that were from a German Taffelwasser. I remove gaskets as soon as they are empty to keep them from stretching out and replace them when it's time to bottle. Nothing but satisfying POPs and frothy goodness. Only problem I ever had with swing-tops was a batch that came from a beer called Jubel. They were like little jugs with a handle at the top like a gallon cider jug. Those were weak and a couple turned into Belgian Trippel hand grenades. Not TOO dangerous cause I have learned to keep my fresh bottled stash closed in plastic totes until it's time to refrigerate - but messy nonetheless. They don't make Grolsh bottles like they used to, agreed, but even the newer seamed ones have held up just fine. If the recipe is ambiguous about the length of time, ie "One to two weeks primary, One to two weeks secondary", I go for two and two and then bottle. Only time I cut it short is if I'm kegging.
**Snuffy