Here's the stats page on wordpress.org: http://wordpress.org/about/stats/

In short, depending on the purpose & features of your plugin, you may want to consider testing on older versions as well. With themes, it shouldn't be too big of an issue to ensure compatibility for a couple of versions back, at least.

Edit: Probably not entirely accurate, see this tweet by a core dev: https://twitter.com/nacin/status/456516848763146240

It's pretty acceptable for themes and plugins to expect people to be on the latest version of WordPress, or at worst maybe one version behind. The more complicated support metric is PHP version. Unfortunately, widely used plugins probably still need to support PHP 5.2, because so many hosts won't upgrade their client's PHP versions.