TeX looks great and all for equations and reactions, but, at least on some browsers, there is a significant break in style between it and the body text. Sure, one is serif and other sans, but the weight is different (namely the body text is hinted) as well as the size. Also note that for users of the SE mobile apps and in google search results, TeX does not render properly in question titles.
Using TeX (mathjax and mhchem) to markup math and chemical equations is absolutely fine, and is much easier than attempted to use HTML sub- and superscripts and essential for using greek letters. Use it -- that is why we have it turned on! Don't overuse it though. For simple numbers with units, it can be overkill. Consider:
1 J s = 1 kg m<sup>2</sup> s<sup>−1</sup>
- 1 J s = 1 kg m2 s−1
If the rest of your post isn't heavy in TeX and all you need are a few numbers with units, consider using the HTML markup. There isn't a huge different in how the text is rendered and it makes the post easier to copy via cut/paste (remember all the content you post is CC licensed cc-by-sa 3.0). As a bonus, the HTML markup will render everywhere for every user, in post titles and in google search results.
This post is a derivative work of this post by Nick T on chemistry.SE