Keep in mind, if you can do your startup as OSS, it changes the game.
For example, look at Monty and MySQL. He sold to Sun, he didn't like the direction they (now Oracle) took it, so he forked his original codebase, and kept developing it in a new company.
There may be contractual reasons why this is difficult, but there are huge advantages to releasing your code if you can.
* Part of how the original Mysql made money was by saying "ah, this is GPL'ed code, but if you'd like to include it in a proprietary work, we can sell you a license for that, since we own the copyright to the whole thing". He can't do that again.
* I'm not sure how often you can burn your bridges like that and have people happy about it.
For example, look at Monty and MySQL. He sold to Sun, he didn't like the direction they (now Oracle) took it, so he forked his original codebase, and kept developing it in a new company.
There may be contractual reasons why this is difficult, but there are huge advantages to releasing your code if you can.