Depends on the collector - there are some that strongly prefer head-seal intact, others who like that a pony's openable, peace of mind knowing there aren't gross things lurking around inside. If the neck ring is cut or torn rather than simply unglued that definitely detracts from their value.
Personally I open all my ponies where possible to make sure they don't have hidden problems. Pretty scary how many show up with hidden tail rust, mildew, and grunge on their insides.