You may not have to, but it is probably a good idea to add a buy-back provision even though any unvested equity would go back to your company because you want your agreement to be thorough and provide for any possible scenario, especially when talking about equity.
For the same reason, and as other answers have also pointed out, I would suggest hiring an attorney to help you draw up your founders agreement. This agreement will obviously have huge ramifications down the road so it is important to get it right at the beginning. An attorney that has experience drawing up these types of agreements will be able to work with you to craft an agreement based on your company and its goals.
Drawing up a founders agreement on your own could be dangerous because you could mistakenly add something that you do not necessarily want in there or you could leave something out all together. Both scenarios would cause substantial headache down the road and thus provide you with an incentive to hire an attorney at the outset so that you can avoid them. Really the only reason not to hire an attorney at this point is to save a little money, but attorneys who do this often will typically do it for a low, flat fee so you wouldn’t be spending too much, especially when compared to the value you are actually receiving in return.
If you are looking for an attorney to help you draw such an agreement, or if you just have more questions, you should check out. The experienced lawyers on our site have helped set up hundreds of vesting schedules. We offer no obligation price quotes & a money-back guarantee!