Welcome to Flyertalk!
In my few experiences flying Frontier, I have never been asked to size my personal item. However my bag has been correctly sized so a casual look by the gate agent would not give her any reason to ask to me to put it in the sizer.
The key for you is that your bag is soft-sided. I would just avoid packing the bag to the absolute gills just in case you are asked to put it in the sizer.
If for some reason every passenger is asked to put their bags in the sizer, keep something you can put on --such as a sweater or the like-- on the top of your bag. That way you can put it on and lighten your bag just until you board the plane. At which time you can put this article of clothing back in your bag.
Don't worry, you'll be fine!