Unless you are sending tons of emails "per day", you DON'T need a dedicated IP, it's only an extra unnecessary overhead.
Of course with a dedicated IP, don't expect that once you start using it your open rate will go through the roof, as in fact with a dedicated IP, there is so much setup work that needs to be done at first (i.e. warming up the IP).
Once you start sending tons of emails per day, at that stage email services (such as google, yahoo, aol...etc) will start suspecting you especially once the spamming flags arise either due to accidental flagging or intentional... Thus just stick to a shared IP, and let the guys at SendGrid do the heavy lifting for you.