How much is a little? The rails are forgiving, but only to a point. It they are pointed off too much they will bind.
To get the holes straight, you can use a drill guide. If you have a 1" x 1" piece of oak, other hardwood, or even metal, and access to the right tools, i.e. table saw or router and drill press, you can make a simple guide. Cut a V groove along one face of the wood that is parallel to the opposite side. That is what will sit on the tube like a piece of angle iron does. Use the drill press to drill guide holes in the center of the V groove to the opposite side. As long as the side with the groove and it's opposite were parallel, you can just drill from the bottom of the v groove. I would drill various size holes so you can use different size drills later. Just set the v groove on the BT and put your drill through the the tightest hole to keep the bit square and drill.