The filter, if not changed, adds non-needed bacteria, lime scale and rust to your coffee. The water tank is holding onto water where bacteria and mold will grow.
The cup holder has holes to catch drippings, but it also holds onto the coffee and heats it up. So, let's keep it clean!
How To Clean A Keurig
If you do not change the filter every 2 months or every 60 tank refills, you are adding in lime scale build-up and rust to your coffee. That surely will change the way your cup of coffee tastes.
The water reservoir also needs to be emptied and left to air dry after each use. Who wants to use water in their coffee that has been sitting in a confined space where bacteria is growing?
Also, the bottom holder where the cup sits usually has holes in it to catch the drippings of coffee. Take that apart and wash thoroughly after each use, as it holds onto coffee and creates bacteria like a Petri dish. ICK!