In my location, you are required by code to insulate the hot water pipes in a crawlspace. Honestly, the insulation for pipes is cheap and you can do the job yourself no problem. It will make a (teeny tiny) difference both in your monthly bills and the temperature at the faucets. Probably not huge (for either) but it's still worth doing.
You actually lose a lot of heat through PEX...that's why it's use for radiant floor heating. Insulating it is a good idea. Since you are going to be crawling around in the crawlspace any way...go ahead and insulate the cold water pipes as well. Cheap freeze insurance. Also take the opportunity to look for anything else amiss...went spots in the dirt indicate leaks...rotten sub-floor or floor trusses, etc. I usually do a crawlspace inspection once a year regardless.
As far as insulating out of the tank.....there is a spacing requirement where code forbids it. 12" or something. Basically don't insulate the pipes coming out of the top of the hot water heater. Check your code for exact measurements.