"They" say to always use the same ruler when making a project -
 

Due to the differences in how thick the lines are on some rulers and ruler placements when cutting -

I was wondering if anyone actually measured their piece sizes when using different rulers? (I am talking about measuring to at least the nearest 1/16th inch - if not smaller increments.)

Mary Ellen Hopkins has mentioned PPM - Personal Private Measurements - and i was also wondering if this concept could apply to cutting techniques and equipment.

I am so easy to mess things up I always use ame ruler or cutting items on a project first step to finished project.

Some rulers have wider lines then others. Line up the inch lines on different rulers to compare. It can really mess up your cutting to swap different brands of rulers.

I have a bunch of different brands and use them all. An inch is an inch is an inch. The only issue, I think, is that some people find certain rulers easier to read, and those are the ones they should use.

Exactly. But if you need to lay blame., rulers won't talk back. :p

I am slowly switching over to just a few Creative Grids rulers because I like the less slipperiness of them so I tend to use the same anyway. I find that between slowing down while I cut, shifting my holding hand up carefully and using them greatly increases my cutting accuracy. All those people in the past I watched swoop their rotary cutters up so quickly really weren't being good role models - at least for me. It's a wonder more people don't cut themselves.

Absolutely. Makes a difference. Used two different rulers on a project and had a mess. Never again. Actually, I find that I have to decide on whether to use the RULER marks or the MAT marks. Yes, I use my mat marks often. Lay a ruler on the mat and look how they line up. You'll see what I mean.

The flip side of this -

If I had three people using the same brand and size/model of ruler - and asked them to cut 4 inch squares - do you think they would be "exactly" the same size?

Of course they would not.