The location, center or side, will make little, if any difference IMO. No need to worry about some wort left behind in the mash tun if you are fly sparging. There will be only a tiny amount of sugar remaining when you near the end of the sparge. This very small loss is negligible. Batch sparging is different in that respect, but you could simply tilt the kettle some near the end of the runoff if necessary. Use a little more sparge water to compensate for any unrecoverable wort left behind. It's not a big deal at all.