I don't particularly see the sense in this. It's never explicitly said that Margaery was in on it, and in the show it's implied that she wasn't. Olenna and Little Finger, if they're as masterful as we're supposed to believe, would not want any more people involved than absolutely needed to be.

I could be 100% wrong on this, and expect to get replies telling me so, but I always believed it was the pie that was poisoned. I don't believe there was much actual investigation outside of Cersei's witch hunt to confirm her assumption of Tyrion's guilt, so IIRC we don't know if it was the wine, the pie, or something else. That said, anyone planning this assassination would expect an investigation to follow. Being a well-designed assassination, that is one of the two key factors that would need to be considered beforehand.

1. How to ensure that Joffrey and ONLY Joffrey received the poison.
2. How to distance conspirators from the investigation itself.

1 - If the poison were to accidentally get distributed to anyone else, the result is a series of shit outcomes that you want no part of as an assassin. Wrong person dies, ceremony is halted, investigation launched, Joffrey still sits atop the Iron Throne, future assassination attempts will be much more difficult to pull of. We need some method of distribution that is targeted both in terms of who consumes it and when they consume it. Moreover, it's a Royal wedding, following another high-profile wedding that was immediately famous for its assassination of Robb and company, and slipping something into the cup seems implausible and unplannable, meaning any wine would need to be served from a previously-poisoned flagon. On the other hand, the one food-related thing that any assassins would know before the ceremony and be able to plan is that the King will be the first to taste the pie. Joffrey is a lot of things, and one of those is an absolute self-centered bombast. There's no way any one else is eating a piece of the pie before he does. Additionally, there's a lot of confusion that could happen with the wine plan, and because there's only a few people whose hands are on the wine, this plan lends itself toward an investigation turning something up (which leads to #2).

2 - Let's say an actual investigation occurs. They find the wine flagon that was poisoned, they could probably piece together which servant had it (this was a hugely public event) and then torture their way up the ladder of command. It's much easier to chase this lead than, say, having to question every single person on an entire kitchen staff who would have baked the massive pie. The pie is prepared entirely behind the scenes, so you don't have to commit the act of poisoning it in front of hundreds of folks. You just need one well-paid baker or caterer who can quietly exit the scene unannounced before the pie is even in play.