Should a peer review for an academic journal include a recommendation for acceptance?
Remember that the final decision is always up to the editor regardless of your recommendation. And (hopefully) the editor reads your entire review, not just the recommendation.

I usually include a recommendation in my reviews, but I always try to justify it thoroughly, so that the recommendation itself is essentially a redundant summary of what I have already said. If I do not have a strong opinion either way, I will say so.