It depends on what the instructions ask for. If <em></em> is expected then use it.
In production (a web facing the internet) we take into account search and accessibility concerns. <em> and <strong> are considered more important than <i> and <b> by search engines. We need to consider if what we are italicizing is a visual matter, or a matter deserving emphasis. Bold text has the same general consideration.
Users with limited or no vision depend upon assistive technology such as a screen reader to “view” our content. If we pepper the text with EM’s and STRONG’s, the screen reader will be yelling at the listener since these tags trigger a change of voice in the SR.
On the whole, we should consult a writing style guide for best practice recommendations. When we only wish to emphasize a word or short phrase, then it is okay to use EM or STRONG, as apply. If an entire passage deserves emphasis then these should not be used. Use a BLOCKQUOTE, instead, to single it out.