Definitely the same brand at a minimum throughout a project. Preferably the same ruler but that's just not always possible. One thing you can check is to pick your favorite ruler. Measure a 1 inch square and put post-its or tape on that inch square. Line that square up with your other rulers and see if the measurements are actually different. It's very possible they are. Also, make sure you always measure to the same side of the line. Err on the side of large - you can always trim down a smidge if you need to.