Actually, it's a little more complicated than that, the recommend tag page states that:
Before using any tag you don't see below, read HTML and Inline CSS very carefully.
If you click the link on the Recommended Tags page, it takes you to: https://www.wikitree.com/wiki/Help:HTML_and_Inline_CSS which states:
Unless specifically recommended against on a WikiTree help page or style page, all wiki markup tags can be considered standard. WikiTree will attempt to support them if it doesn't already. We will publish a specific rule recommending against them if necessary.
The underline tags are standard Wiki markup and can be seen by clicking the link to the Wiki markup cheatsheet on the Recommend Tags page: http://en.wikipedia.org/wiki/Help:Cheatsheet
I would recommend reading through both of these pages. We should keep older profiles as simple as possible and stick to the recommended tags. This is because we share our ancestors with so many other people, and everyone has preferences which makes style arbitration difficult.
But as the guides state, for your personal profile and free space pages, you are free to do what you want.