Sage words Ms Whips. Or as the phrase goes - be careful what you wish for, you may just get it.
I don't think it is important to live out your fantasies as such, it is nice to do that, but important no.
What is more important is having strength in your relationship, whether it's conventional or a swinging one. If you have that then it allows you to explore the fantasies without, hopefully, causing any problems.
I think discussing fantasies is a great idea and I think is the crucial first step to actually deciding if you want to live them out or not. You and your partner may have very different ideas of what they want and perhaps your fantasies are not compatible. Say, you want a FFFM 3 some and she wants to be gangbanged by 5 hot studs while you watch on wearing a dog collar.
I have lots of fantasies for different scenarios, people, places, acts, moods etc. I've even committed some to paper to foolishly send out to people to laugh at and then pretend they enjoyed it. Of them we've probably done less than 1%, but it doesn't matter, we've talked about most of them and that can be just as fulfilling.
And there is always the very good point that others have already made, the fantasy may well be a great deal better, than the reality.
Live out your fantasies, you only live once and what you dont try is gone when your dust. If a relationship cant handle one fantasy gone wrong then it was never very stable anyway (thats not aimed at anyone in particular btw)