When I was an undergrad, I had instructors who did not tell students that there was more than one version of the exam until afterwards. I also had instructors who told students that there was more than one version, before the exam started. The question is: should an instructor tell his/her students before the exam that there are multiple versions of the exam.
Why would knowing in advance matter to anyone except cheaters? This information will not help honest students prepare for the exam or change how they approach it, so for them announcing it afterwards seems just as good as in advance. One argument for announcing it in advance is that some potential cheaters might decide not to try to cheat (and spend more time studying or become a little more honest), but the counterargument is that it might just encourage them to find more effective methods of cheating. I don't consider either of these arguments especially compelling, and I'm not convinced it really matters much whether the announcement is before or after the exam. This issue tends to come up in large courses, which are often pretty standardized. I'd bet the students who care whether there are multiple versions of the exam can easily find out what this professor has done in other large classes, so there usually won't be much of an element of surprise.
I think the answer is "Yes". This causes that low number of students considering cheating on the exam to forget it and put their time towards studying for the exam.
And for other students it would make no difference whether there is only 1 or multiple versions of questions as long as the difficulty of the versions is fairly kept the same.
Update: Following comment of the author about keeping the same level of difficulty among different versions, I still believe that the students have the right to know that there are multiple versions of questions and as an added option you can even tell them that the only difference is on the order of question. That does not make any problem in my opinion.
What I do is to make the versions of the exam different colors, but not say anything about it (as Kimball describes in a comment above.)
Here are the main alternatives, as I see them:
Hide the fact that there are multiple versions of the exam.
Mention in advance that there will be multiple versions of the exam.
Disadvantages of option 1 are: (1) it's hard to distinguish the different versions when handing them out and when grading them, (2) students might still try to copy from their neighbors' exams, which is distracting (although I suppose this is actually an advantage if you want to maximize the total number of students you catch cheating.)
Disadvantages of option 2 are: (1) it is an unpleasant reminder of the possibility of cheating (as Anonymous Mathematician mentions in a comment,) and (2) students who were planning to cheat may adapt to some other method.