I think it depends on the setting. If the setting is France and the languuage being spoken is English, then they should just use their natural accents. A french accent would be cheezy in this situation. However, if an American actor is playing a French person speaking English, then they should use a French accent. Or something to that effect. They are actors after all, aren't they? Let's see some acting. I don't want to see them act like themselves.

Here are some bad examples of what I'm talking about:
 - Leonardo DiCaprio in The Man in the Iron Mask
 - Kevin Costner in Robin Hood: Prince of Thieves

I was embarassed at how poorly they did these roles. It was almost comical to hear their plane old American accents.

Good examples:
 - Kevin Kline in French Kiss
 - Gwyenth Paltrow in Possession.

I hope I'm making sense...