The answer to this question is that it really does not matter! T shirts or formal suits, they are both ok. However for investors we would recommend something more formal and also for press at the end of the day its not about what you are wearing its about the company you are trying to build!
Do as you wish!
Since you are not Mark Zuckerberg and the chances you are going to build the next facebook are so so small we would recommend wearing a shirt for your investor meetings. It may help you close to deal and secure the funding, but having been through techstars and seen startups dress codes do not really matter. Your education and your links to the business you are trying to build are a LOT more important.
For your day to day
T shirt and anything that makes you feel confortable is probably the best! There is abolsutely no need to dress formally but of course you can do so if you wish! So weather you are trying to close pilots with corporates for a startup or trying to do anything to get your next customer its really up to you.
Out of the 100 startups we spoke to, only 2% really had some kind of dress code. The earliest ones didnt have any and most people turned up in t shirts and any way they wanted to dress!
EDUCATION AND BACKGROUND ARE MORE KEY
For VC's your education and links to the industry will give you a higher chance of securing funding from any investors!