the way git works is that a branch name is just a pointer to a specific commit. Once you merge a hotfix branch into master, your hotfix and master will point to exactly the same place in the commit tree. As you make more commits on master, the hotfix branch will continue pointing at the same place while master will get updated. Your history will always be preserved.
So basically the only reason to keep hotfix branch after a merge is if you plan to make any more changes to the same hotfix, which doesn't make much sense once you release the hotfix. So you should feel perfectly safe deleting the branch after the merge. One more thing you could do though, is once the hotfix is merged, create a tag on the master branch identifying that point as the hotfix release.