I have been attempting to quilt for about 3 years (sewed since junior high) and my cutting still isn't even close to perfect, nor do all of my seams come out 1/4, or even straight sometimes! (BTW, I like the idea of using shelf liner under the ruler). One thing I've noticed is that the 1/4 inch on one sewing machine never seems to be exactly the same as on another machine, and that tiny bit off can mess you up when you are doing a big quilt! So try to do it all on one machine. If you can't (I usually use one at home and a different one for classes), then you have to learn how to fudge it. Some teachers will help you, others want everything perfect.
So - just have fun! Non-quilters will never know the difference, and quilters won't mention any imperfections they see because they have all been there themselves!