Do I need the grate on the bottom of my cage or can I remove it?

You can remove it if you wish.  I leave it in and put paper over it to catch the poop.

I had a Severe Macaw that had a cage without a grate, It liked to play with balls and other toys on the bottom of the cage. After finding out the bird would tear up the newspaper I was using on the bottom of the cage to build a nest and laying eggs non-stop I had to stop using newspaper. I unfortunately switched to corn cob to replace the newspaper. Everything was fine for over 35 years and then the bird got sick and died. I tried to save the bird and found an excellent bird Vet who had a blood test done and found out the bird had a liver infection. The Vet told me to never use corn cob in a bird cage because if the corn cob isn't died properly the bird can catch diseases from it. You might want to keep the grate to keep the bird from getting poop in it's feathers.

Do YOU need the grate at the bottom of the cage? No, but the bird inside the cage may need it to protect itself from gaps that trap a leg, their head or beak.

Can YOU remove it? If it's removable, sure why not. If you are forced to alter the cage to remove the cage, you will need to decide if buying a cage without a grate, might be a better idea. Just make sure there are no sharp edges or gaps that can harm the bird. So using common sense is something you will have to exercise. 

Depends on the construction of the cage and whether there is anything below the grate that would be hazardous for your bird. Many have larger gaps in the wire, sharp edges, or other design features below the grate that would make removing it dangerous for the bird.