He thinks it is a smart thing to do.
He thinks it were a smart thing to do.
He thinks it would be a smart thing to do.
The first is a simple statement "A is B".
The second is incomplete, since the were construction is usually part of a "if-then" conditional:
He thinks if it were a smart thing to do ... (everyone would do it).
The third is implicitly incomplete, since would is used in hypothetical sentences, usually with an "if" conditional:
He thinks it would be a smart thing to do (if he only had the time)
It's not absolutely necessary to explicitly state the condition where it is true, but by using "would" you do imply that there is some condition.
As with anything, which you choose to use depends on what you want to say.
I think you will vote in the election.
I think, if you were to vote in the election, it would be good.
I think you would vote in the election, if you thought it was a good thing to do.