Steemvoter only requires a POST private key, and that's fairly safe to give out.
I would alway check out any tool before giving it access to my account. I think in this case it's a well known member of the community that runs this site @penguinpablo, so there is more trust that way..

There are other tools that want more access, using an Active key. I spent a lot of time researching that to find it's usually OK to give out credentials, if there are discussions and other's have checked out the security for the community at large.