Printing accuracy, on any inkjet printer, is almost always better in the horizontal direction (direction that the print head moves when the paper is stopped), than it is in the vertical direction in which the paper moves. The reason for this is that paper feeding motors are not as accurate as print-head positioning. So if the block you printed at 6″W x 6″H comes out 6″W but not exactly 6″H, it most likely is caused by your printer’s paper feed. Each printer also has a margin of error range in the paper feeding direction. Each printer has an internal self-test page that, when you know the secret formula, you can measure the paper feeding error to see if it is within spec.
Each model of printer will likely be different in terms of paper moving accuracy. Inkjet printers can be very accurate at moving paper, but for example in many of the inexpensive inkjet printers, the parts used to build the printer may not have been selected to move paper with high accuracy. This may or may not be the same case for LaserJet printers. Since each model printer uses different components the only way to determine the printing dimension accuracy is to test it out by printing a graphic with known dimensions and then measuring it.