I quilt with rulers on my Janome quite often. It can be tricky to get used to it, but very do-able. There's a ruler foot for Janomes (for use on a frame system, but I use it on my stationary machine) that really helps reduce the chance of getting the ruler under the needle which is disasterous. There are several posts on my blog (
www.freemotionquiltingadventures.blogspot.com) that show how I do it. There are even a few videos!
Using a long arm ruler is great for cross-hatching!
I need to get a few more rulers to try, but I have a straight ruler, a double curve ruler by Rhonda Beyer (both available from Quilter's Rule) and a few odd ball rulers. Straight lines and gentle curves work best, semi circles for continuous curves and pumkin seed patterns work well too. Having good non-slip spots on the ruler is essential!