For normal cycling purposes Andy, it isn't particularly important but for mudplugging or the kind of high-speed downhilling that some like to indulge in, there may be different requirements to do with grip and balance that don't apply in leisure/utility usage.
For fairly heavy machines that are limited to c15mph power assistance, on-the-limit handling characteristics are hardly applicable so I wouldn't select an EAPC on the basis of battery position or weight distribution over comfort and price, for example.
Personally, the downtube- mounted battery is my preference - I have two models so equipped but there is nothing wrong with those behind the seat arrangements and I have one of those. Many of those make for a slightly longer wheelbase but that is rarely an issue.
Self-builders quite often have bespoke arrangements with backpack batteries, front bag or basket, crossbar bags and so on but that's a very personal choice and unless you are going down the road of self-build, you can discount that kind of thing.
Tom