I see anything with a "C" title as primary responsibilities include architecting a plan and building a team to execute. Early stage startups require marketers who are willing and able to get their hands dirty. There are really only 3 things a startup needs early stage.
1. Be found on search
2. Tell an audience who cares about the problem they solve, that their solution exists.
3. Convert traffic
There is certainly a lot that goes into these buckets, such as, content strategy, strategic partners, social and PR strategies, product conversion, etc. The point is, if you want to join a startup you should ask yourself 2 questions:
1. Do I care passionately about the problem they solve?
2. Am I prepared to work for less than market value or no salary for a period of up to 12 months?
If the answer is yes to both of these questions, than you want to think critically about the value you bring at this stage, which should align with the 3 critical marketing objectives highlighted above. The answer to your question is, you want to dive in immediately. Even if the product isn't market ready, you can build customer personas and test customer behaviors.
If the answer is no and you don't have the ground level skills to make meaningful traction in the 3 critical areas, than you should pursue companies who have raised at least $2M+ series A. I'm not sure the real benefits of this though. This middle ground still presents significant risks with much less upside of the earlier stage company. Basically, you'll get a salary with a small <.5% piece of stock. Whereas the early seed stage concepts, you can take a meaningful chunk of equity as compensation which gives you the upside. It all depends on your skills fit and appitite for risk.