Iuse what I call an invisible stitch. It is sort of like a french knot. On my french knots you go up and down in the same place. So for a tied knot I go up and down 3 times in the same place being careful not to go in the exact same place or it will pull out but keep as close as possible. Then I have the tail on the back side and after I get the 3 I run the thread through on the backside to catch the tail like you would on a button to secure it. Then after I have made sure the thread is secure I clip the tail as close to the knot as I can without cutting the knot itself. It won't be totally invisible but it won't show much especially if you use a thread that matches the quilt. Make sure when you sew the first time you catch the beginning tail in that stitch and also in the next 2 stitches. Then you only have to deal with the thread on your needle at the end. I guess it really isn't tied but it gives the same effect with out any tails showing.
You can do the same 3 times through and tie it at the end instead of hiding the tails and then after you tie it a couple of times you can trim the tails close. It is whichever look you like.