I've had several near misses in 2500 miles of electric riding since 2007, but only had to push once! It's good exercise, I used to do it a lot with my 1967 Triumph Bonneville, too.
More seriously, if you monitor your battery state (I just use a voltmeter, but there are more sophisticated ways of doing it) and your odometer, you should not be caught by surprise. My first conversion has an on-board charger, so I can mooch an "opportunity" charge just about anywhere.
Another consideration of course is to build a conversion with enough battery capacity for the riding you want to do. My bikes will get around 10 miles per kilowatt-hour capacity in city riding, but a bit less (maybe 7) on the highway.
As to self-discharge, it depends on the battery types. Lead batteries self-discharge at a measurable rate, and in my experience the nickel-metal-hydride batteries self-discharge at an atrocious rate. The ThunderSky lithiums I've used have an almost insignificant self-discharge rate, and the lithium polymers I'm playing with now also seem to hold their charge very well over time.
