Actually I'll clarify my previous post a bit.

I'd try keeping the centre of gravity as near to the rails as possible, to minimise how much the gantry trys to rock under acc/dec (think of the motor as a big pendulum trying to pivot around the rails).
However, given that your gantry is quite 'substantial', it probably won't be as big an issue for you, than for somebody using a lightweight alloy extrusion gantry.