This was also discussed in a few previous threads - and usually devolves into arguments about which to use i/em b/strong. There is not a specific button for em/strong like there is with i/b — BUT — you can make a …. Wait for it … CLIP!!
Then you can set those in your clips toolbar. This is what I use:
Name Clip
EM "<em>\1</em>"
Strong "<strong>\1</strong>"
Just highlight the phrase and click the clip and it will surround the highlight with the tag.