Is it necessary to submit papers to IEEE conference which are PDF Express Plus approved?
PDF Express is basically an automated means of making sure that you didn't mess up your document preparation in a way that will later become a headache for you, the conference organizers, or the IEEE.  For example, you might have things misconfigured such that you ended up with A4 instead of US Letter paper, or have some bad fonts in there.  If you're using the standard IEEE LaTeX templates and not doing anything particularly creative with your formatting this shouldn't happen, but even then sometimes it does.

The IEEE camera-ready sites will typically tell you that you are required to check beforehand, but de facto allow you to submit.  This is sometimes very important, because sometimes PDF Express isn't working (we had an issue of this type with a conference earlier this summer).  Checking with PDF Express is a good practice to follow, however, because it's a few seconds of insurance against the possibility of a whole bunch of headache for yourself and others down the line.