it's best to not ask the host and swappers to change the rules. you'd be surprised how much confusion and mayhem even the simplest change could create.
unless is a partnered swap and you've been able to verify that your partner has no allergies or other objections it's always safest and most courteous to avoid sending anything scented. do your best to remember that. safety first whenever humanly possible.
the flip side of that is that swappers with allergies should not throw fits or say nasty things about those who do forget and send scented items. it's unfair, unkind, ungrateful and ill-mannered. after all, they are adding what they think will be a special extra touch. if an accidental kindness puts you at serious risk then you should either exercise extreme caution when opening packages or not participate in swaps at all.