Yes is makes a difference Ecspecailly if your going to a bigger tire. If you can lay your stock rim on the ground and measure through the hole in the middle to the ground this is your offset. you may need to use a straight edge if you have tires on the rims.
The reason it is important is, the further you off set the tire from the hub the bigger radius your turns on. This will push your tire into you floor board and fenders. it will also increase your scrub radius which makes your bike harder to steer. The long term effects will show up in your ball joints and tie rod ends.
long story short.. if you are going with the same width rim keep the back spacing the same. If you are going with wider rims you need more back spacing. try to keep the front rims so that the hub is in the center of the wheel. Hope all this helps, Craig
