I like to keep it below 65F. Preferably starting in the upper 50s and letting it rise to 62F or so.
We did a tasting of hefes at the NHC in San Diego that were fermented at different temperatures and mashed differently. The best flavor came from the one fermented cool (62F) and with a single decoction. Harold Gulbransen made them; he's also medaled in this category in the past. He knows what he's doing, especially in this style. So I'm convinced he made all test beers equally well.
I was on the tasting panel, and the difference was fairly clear. A warmer ferment gives a less clean flavor. It still tastes good, but it doesn't taste best. If you can't control your fermentation temps, try brewing it during a different part of the year when the ambient temperature is more suitable. The warmer one had more banana but it also had more of other flavors; the increased esters added an acidity, it seemed, including an apple-like flavor. Without running an analysis on the beer, it's hard to say what all was in there, but I just know what I tasted.
I'm convinced cooler fermentations produce cleaner tasting weizen beers. Don't misinterpret that statement. I can see some people saying, "but I don't want cleaner, I want banana and clove". Meant cleaner relative to that yeast. At cooler temps, you still get banana and clove. You just don't get other things. At higher temps, you do (or more appropriately, you increase the odds that you will).