Lately it has become almost a hidden secret, something we are reluctant to admit. Yes, it is ok to use stencils or templates in our free motion machine quilting!
The clamshells, above, are small, maybe 1/4" or 3/8" in size, and yes, I did them freehand, with no marking. Sometimes a straight "horizon" line to help me stay level can be added, but grainline in fabric or a line of piecing usually is all I need to keep them nice and even, in a classic, traditional style. Funkier clamshells, "relaxed" clamshells don't need to be level or the same size, so go for it with them, with no marking.
However, if you have difficulty getting them even, and want them to be even, don't forget about a template. Anita Shackelford has a wonderful mini and larger template for perfect clamshells and for larger fan designs that works great. She has many photos for you to see the applications for this template, including a Sashiko design based on clamshells for machine quilting.
Click here to go to the page on her website, http://www.thimbleworks.com/ to see the examples using this tool.
I like the perfect spiral tool as well, and of course infinite feathers if you need some help getting that feather shape established in your brain. Mark some, quilt them, do it over and over until you can do it with minimal guide markings and no marks eventually.
Remember, it looks easy when you see others quilt feathers and such so effortlessly with no marking, but if you need marking and templates, it's perfectly natural and a very good thing to have in your repertoire to get those quilts done beautifully.