The only way I've found to counter the problem of wonky cutting is to start with a piece of card stock larger than you need. Make a score line again slightly longer than you need, fold and cut through both layers with a ruler and either a stanley knife or other sharp blade. that way the card is perfect.
Unfortunately manufacturers don't make precision cutting or measuring tools for the craft industry although they charge the prices

When making measurements many professionals first cut a small piece of to get a true straight line, then use a set square to make a small mark to get the next straight line and when measuring make sure you use either a mechanical pencil with a very thin lead or better still a pin point. Using a normal pencil can leave your line width out. After a while of doing it this way, it actually becomes quicker than getting out a trimmer and messing about to try and get a straight cut. (You can tell I'm a bit a**l about wonky cards lol)