There's a great 1/4" foot for the Janome that will give you a 1/4" seam, although you should of course test it on your machine. It's the Clearview Quilting foot and guide set and you can order it here:
http://brubakerssewing.com/store/ind...guide-set.html or of course get it from your local dealer. Brubaker's is an awesome shop and ships right away and has good prices.
The reason I like this foot is that it is see through and the flange for the 1/4" is removable. If you use the foot without it you line up on the edge for 1/4" seam, and it also has a marking for 1/8" seam. It also includes a stitch in the ditch flange. I no longer have a 6600 (I have a 7700) but I use this foot all the time on my Janome class machine and I love it.
Another option is to use the 1/4" Accufeed foot. You will have to move the needle over considerably. I used to set mine at 5.5, but I know others with the 6600 who had to set it closer to 6.0. You may find it easier to sew with Accufeed and if that's the case then you can measure where your needle should be set and set that as your default setting when you turn the machine on.
There is a wonderful Yahoo group for owners of the 6500/6600/7700 series of machines; very helpful on these types of issues!