Yeah it does matter. Nearfield monitors should be upright, and level with your head. When you're sitting in the "sweet spot", your head and the two speakers should form an equilateral triangle. You want to check that the speakers aren't too close to the walls either, otherwise you end up bass heavy, but that said you might want that, so experiment. Check that there's no major reflective surfaces interfering between the speakers and yourself, so you don't get any extra highs bouncing back into your sweet spot. You might also want to try some DIY room acoustic stuff.