Do I need isolation footers for my components if I'm already using an isolation rack?

I do. You can try a few mid-line varieties and see if you hear a difference. I have a very revealing system with dedicated lines, power conditioning, quality cabling and acoustic treatments. Everything matters.

Do I need isolation footers for my components if I’m already using an isolation rack with de-coupled shelves, such as the Mapleshade racks?

You do. Sort of. But really, there's no such thing as isolation.

What there is instead is vibration control. Most of the vibration actually comes from the component itself. Its the way that vibration is controlled that affects the sound as it gets into the music. 

That is why no matter what rack or shelf or whatever you use, even the floor, still and always whatever is used under - or on top of! - the component will affect the sound. For better or worse. Depending on what is used. Everything makes a difference- but not all the differences are good. Or good in the same ways.

Do you "need"? No. But since this has at least the potential to be a very cost-effective improvement, its well worth the effort to try and see.

What rack are you using??

Here is an easy exercise, to answer your question.
Gather household items: rubber doorstops, protective furniture leg discs, pencil erasers, pill bottles, wooden cubes, different
kinds of tape.

Place them under and on top of: electronics, cables, 
ends of power cords, speaker cabinets.  You can even use them in some combinations, together.

Some will improve the sound, some will make it worse.
The piece that improves what you hear in one application, might not work in a different one.

This is a harmless, low cost way to experiment with tuning your system.  The higher the quality of your setup, the easier it will be
to hear changes.

At some point you may want to make an investment in
products designed for these applications.

Make it fun.