Jeans and t-shirts are OK in a lot more places than they used to be but on the East Coast or in many other countries dressing so far down is disrespectful. If you are under 25 a hoodie is not out of place but for anyone older a hoodie looks juvenile. So do Vans. And try getting into some of the nicer restaurants in New York or San Francisco wearing a t-shirt and hoodie. Unless you're Mark Zuckerberg.
If you are going to wear jeans in a professional context you should dress them up with nice shoes, a dress shirt or button-down, and a blazer or sport jacket. And not gray. Gray jackets are for car salesmen and floor managers at Car Toys. If you only have one jacket it should be navy. Think Richard Branson, but if you're not Richard Branson don't expect to be taken seriously dressed like you don't give a shit.
Clothes don't make a lot of difference if you look dirty or poorly-groomed. The dirty hipster look is fine for watching an indie band at a faux working man bar swilling a PBR, but not at many other places. And go easy on the tattoos and piercings and big rings in your ears. Err on the side of caution when you only get one chance to make a good impression. Your business plan and elevator pitch count for nothing if you immediately offend your audience or distract them with your affectations.
You might think it doesn't really matter, but you won't know when you're not being taken seriously or laughed at because people who have manners won't say anything.