my juki f600 defaults to 2.4 but i find that i turn it down to 2.0 quite frequently

I use a locking stitch at the beginning of a seam and then 1.75. I end the seam with a locking stitch too. I press most seams open. My seams are not coming apart unless I want them to!

I also use 2.5 and have never had any issues. I lock my stitches at the beginning and end. I also mostly press my seams open; otherwise they are always going in the wrong direction.

Mostly 2.5. No problems so far.

Ditto on the 2.5. Sometimes even higher. Never had a problem with it coming out. And when you have to rip which I do more than "occasionally" the thread comes out easier.

I use 2.2. If the fabrics are light and dark, and I'm using a light colored thread, it may show through at the dark fabric. If my stitch length is a little smaller, the thread color doesn't show through.