I had a ruler that was aupposed to be 6" wide. Everything I cut with it was too small. I finally put it up to several other rulers and it was on 5 and 7/8". It went in the trash.

I now use the same ruler when cutting out a quilt and when I start sewing, I use the same machine till the end of the piecing. One of the machines I piece on is more difficult to get that scant 1/4" seam, I know it's me, but I still have a problem doing it.

Honestly I've never had a problem using multiple rulers OR multiple machines on a project, and I do both frequently. Not sure if I'm lucky, extra careful, or just so inexact in everything that I just don't notice, but it's never been a problem for me. I don't understand why it would be, as long as your rulers aren't made wrong (and haven't been shaved down from wear) and you take the time to check your seam allowance before you get going.

I suppose if you have a really worn out machine your settings might somehow "wander", but that's a separate issue. My travel machine is always kept set up for a quarter-inch seam, but I check it periodically anyway - I think that's just a good habit to be in.

I've checked my rulers in the past and they all agree on what an inch is, and so do the lines on my big mat. Maybe some mats expand and shrink with temperature changes? I think I'd be inclined to return my mat if the markings weren't accurate. I haven't checked my smaller travel mat, but I almost never use the lines on that one so it kinda doesn't matter. I'm curious now, though.

The lines on one of my June Tailor pressing things - that has the cutting mat on the back - aren't "right."
So I ignore them for measurements but do use them for "squaring' when I am pressing on it.

I'm not particular about which ruler I grab. So far, no issues mixing brands. My problem is making quilts in a group. In our church quilt group, only two of us are willing to sew. We do not have the same 1/4". I use a screw on metal dam set where i want it, the other uses her presser foot which to me is wider than 1/4". It makes it hard to work on the same quilt. I know I am the one who could change most easily, but when I draft a pattern and do the cutting, I use 1/4" seam. That would be much harder to change.

Machines? I have three set up in my family room, two are used interchangeably for piecing and I have no problems. The same with the one I left at the church. All three have the little metal screw on guide. The Elna isn't allowed to quilt, but she does everything else.

I definitely know what you mean - I worked on a group project once - and that was enough to "cure" me.