I can't speak to how much of a problem it is going to cause, but obviously there is going to be more pressure exerted on one facet of the tank if it's not level. Measuring can be a rather difficult way to find if something is truly level, since if the floor is off in the slightest, then your measurements will be as well. I really wouldn't trust a measurement of that sort for anything more than a rough idea.
What you might want to try is a water level. I'm trusting that you have a gravel vac with clear plastic tubing here, so forgive me if I'm wrong. You can either remove the wider section of your vac from the tubing, or simply allow the water to drain until it's all contained within the tubing, but the idea is to avoid the valve found on most gravel vacs that keeps the water from back-flowing into the tank (this will invalidate any attempts at measurement). Armed with your water filled clear platic tubing, arrange things so that the water level on one side of the tubing is level with the lip of your tank. If your tank is level, the water level on the other side of the tubing will line up as well. If the second side is low, your water level will be high, and so on. Basically, the water establishes itself as level no matter how you hold the tubing, so you're effectively drawing a level line (between the two ends of tubing), and measuring the amount your tank is out of level relative to that.
While not doing much to address the problem, it will at least confirm for you whether or not your tank is level. At any rate, I hope that helped, feel free to ask for any clarification.