When using a Smith Machine for bench press or any other exercise that requires a bench, it is best to be sure your bench is perfectly centered. If you have it too much to one side, your lift will be off center and you will end up using one muscle more than the other to lift the weight. For instance, if you are using it for bench press and your bench is more to the right side, you will end up using your right to do more of the lifting. This can cause both symmetry and strength inbalances, and can also cause injuries.
Countless times I have started my bench press on a Smith Machine, only to stop and adjust the bench one way or another. Sometimes I have to stop and do this more than once. I always wish they had a numbered grid on the floor, so you could get get it in the exact centered position, but I am yet to see that.
A quick, easy way to be sure the bench is centered instead of eye-balling it like I always have done, is to simply lower the bar all the way down to the bench before you start the exercise. Take a look at the picture and you will see the knurling on the bar for your grip. You can use the knurling grips that are on the bar to get the bench in the exact center position. As you can see in the picture, the bench is almost perfectly aligned left and right. If I had the bar at the top which is the starting position in most cases, it is very difficult to tell if it is centered or not. Once the bench is completely centered, then you can bring the bar back up to your starting position, rack your weight and start your perfectly centered and balanced lifts. Easy enough, right?