I've jump started mine before at a gas station when my voltage regulator stopped charging the battery. I was out of options and had no choice. You can get away with it if you are extremely careful, but get that positive clamp touching the frame and it can get real expensive real quick, as in a blown up EHC.
Here is a battery maintenance primer written up by the dealer here in St. Louis on their website:
Battery Maintenance
Let me start off with saying that the December 2008 issue of American Iron Magazine has a great article about this very subject. It will go into a much more exhaustive/technical explanation that what we will here, but I wanted to discuss a few things that will help in keeping your Big Dog happier.
1. Battery Tender. I cannot stress this enough. These big inch motors demand a lot from a battery when she cranks over. Big Dog’s new 40 amp charging system does an outstanding job re-energizing that battery to full charge, but sitting in a cold garage or for a long time without riding will begin to erode the cold cranking capability of the battery. A battery tender puts a nice trickle charge on it and keeps it to it fullest capacity. Without a battery tender, I see battery’s lasting about a 1 1/2 years. Now I know, your buddy’s Harley has had the same battery for 20 years, but to steal a phrase from the past, “This ain’t your father’s Buick.” A 4″, high compression engine makes a battery work overtime. I tell most of my customers, the time your old battery will give up the ghost is when you’re in Defiance, MO and it’s about 1:30am. No battery tender, replace the battery each Spring to be safe. Or for less money, buy that battery tender and you’ll be amazed how much more service you’ll get from that battery.
2. Starting the beast is very cold weather. In short, a cold battery does not have the cranking amps that a warm or hot battery has, no matter how old/new the battery is. Warm battery, high cranking amps. Cold battery, she’s going to crank slowly and sometime not at all.
3. Jump Starting. You cannot be too careful when doing this. One wrong slip with the positive cable gets very expensive. Mucho care must be taken, and I like to always have someone helping/holding the cables securely. Nuf said.