That's entirely your prerogative, friend. But, selling it to buy STEEM is a little counterproductive. SBD are simply I.O.U's that mean "I will pay you $1 worth of STEEM." So, if you want to exchange SBD for STEEM you need only go to your wallet and select the "convert to STEEM" option.

But, please be advised that every time SBD is exchanged for STEEM, more STEEM must be created, thus, inflating the currency. With small amounts, this will have very little effect. But, with large amounts of SBD being converted, this can certainly be a problem. This is why many people use their SBD to promote their posts, as it burns the SBD and alleviates some of the debt, and thus the pressure on the system.