A startup where the co-founders have time to work on random OSS stuff is probably working on the wrong things. A startup where employees can spend 20% of their time hacking on OSS or side projects is likely doing just fine. I would be more worried about a startup that expects or demands that employees work 40+ hours. It either means that it's being run by suit monkeys that don't understand how programmers work or has poor project management that requires a constant crunch mode.
