You want to increase the mash temp to 168-172 after the mash is complete (after the time at mash temperature) in order to stop the conversion (as already stated) and then give the grain a rinse/sparge. If you're using a cooler mash tun, that can be difficult, depending on your method. You could heat/boil part of the mash and then add it back to increase the temp. Or add water at a higher temp to increase the mash to 168-172F. Personally, I simply heat the mash tun (a converted keg) to get the grain bed up to sparge temperatures. Then I simply run out the mash wort (or first wort) while adding the sparge water (already at the right temperature, from another kettle) to sparge.
Personally, I get better mash efficiency this way.
For the record, I have used cooler mash tuns before, but found them troublesome. I'm much happier being able to direct fire the mash tun to maintain it's temperature. You can also run the wort from the mash tun, through a HERMS/RIMS system (coil in another kettle with water at the desired temperature) to increase/regulate the mash temp.
I wouldn't use a 5 gallon mash tun for 5+ gallon batch sizes. You're seriously limited for what you can mash that way. IMO/IME, better off having at least a 10 gallon mash tun so that you're able to brew bigger beers when you want (without resorting to adding a bunch of [more expensive] DME).
