This is purely opinion, but it’s based on a semantic preference. Since WordPress has set the comma as delimiter rather than the space, they support the space, as you note, within multi-word tags (unlike, say, del.icio.us, which requires the kind of unnatural ligature options you mention). Given this, I would go with ease of comprehension and navigation for the visiting reader, and space and punctuate exactly as if you were writing normal language. Your readers will thank you, and your tags will be about their content, not about their form. Best, M.