Take a read through this (not my content just something I have read):
http://rob.by/2013/negotiating-your-startup-job-offer/
The key is understanding how equity works and how you can benefit or get hosed with it. A well funded seed company should be paying employee's a reasonable salary along with equity, a Series A should absolutely be nearing if not at market rates. Always ask for the details, burn rates, how much money is left in the bank, expected next funding date, what's the biggest risk, is someone focused on funding full time, total option pool, what debt financing or debt equity exists etc. You need to understand it to know if what you are being offered is fair or not, and there really is no way to know that without having a fairly complete picture, since details will vary so much from deal to deal. Anyone who balks at giving you the details I'd say no thanks.
Also, make sure you know the difference between early hire CTO and a true co-founder. A true co-founder will not be treated like an employee in terms of equity or salary, so make sure you evaluate really what they are offering. Essentially, as a co-founder you would have a far smaller salary early on, but your equity would be more. A co-founder isn't someone with < 4-5% pre-dilution equity IMO. Although others may disagree.