No, you don't need to do it at all.
I seldom use it, only if I need more explanation than the caption allows or when rewriting a caption on a news upload a few days after the event is topical, because the caption is searchable.
So if I wrote, "Crowd waiting on Capitol Hill for President Obama to arrive to give a speech on the Campaign in Syria" in the caption, the file would show up in a search for e.g. Obama, President or Syria, when none are actually in the file. Word is that false searches affects your Alamy Rank, hence your position in searches.
So I might leave my caption as "Crowd waiting on Capitol Hill" and put the rest of the information into the description field, after a few days.