I agree with what Al and Dog have to say, and would like to add another few pennies to the pot here.
The human ear tends to perceive higher frequencies as being more "directional" than lower frequencies. Low frequencies tend to naturally fill a room whereas high frequencies tend to focus a bit more (until they bounce off of solid surfaces). For this reason it's technically more critical to have the tweeter in-line with the ear than the woofer.
This is sometimes why some prefer to put their monitors on their side (with the speaker elements lined up horizontally); if the ideal mounting locations for the monitors (see below) happen to be a bit above head level so that if mounted vertically the tweeters would be well above head level, some like to then mount the monitors horizontally so that the tweeters aren't that high. Additionally, some like to mount them horizontally in order to get a wider stereo field in the high frequencies (by laying the monitors with the tweeters on the far sides from each other.)
All that said, however, a few inches either way isn't going to make a huge difference in that regard. I have the same kind of monitors that Al does and I have mine sitting on a desk overbridge sitting vertically with my ears even with the center of the enclosure (which is just about the top edge of the woofer cone). The center of the tweeter is maybe 4" above my ears and I could care less; it works and sounds fine to me
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Far more important is the position of the speakers within the room, and this is going to have far more effect upon the sound of your speakers and of your mixes.
In general, you'll want to try to keep your speakers (and your monitoring position) out of room corners. Doing so is going to mess up your bass big time. It's instead recommended to place the monitors symmetric to the middle of a wall, with one of the long walls of the room being preferaable to one of the short ones (other factors such as ceiling slant, window location, etc. aside.) Also, it's best to have the monitors a minimum of several inches and preferably a couple of feet away from the wall behind them.
And then there is the issue of first reflections. Sometimes one monitor position can give you problems because of ealy reflections off of the desk surface in front of you. Often moving the monitors just a few inches either horizontally for vertically from the "theoretical ideal" of an equaleral triangle level with the ear can make all the difference in the world because of real world obstructions and reflecting surfaces like mixing consoles and computer desks.
So start with the described ideals and preferences - or at least as close to them as your room will allow - and, if necessary, make fine adjustments in speaker orientation or position as described to get what you need.
(We haven't even talked about room acoustics and treatment, which is a whole other ballgame.)
G.
