If you use body art quality (BAQ) henna, there is no worries about using before or after chemical color. The issue is with henna adulterated with metallic salt. These salts react with chemical color and your hair can literally melt off. Not pretty.
On a related note, if you dye your hair with a combination of henna and indigo, be aware that it you bleach if afterward the indigo will leave a green tinge on your strands. Henna'ed hair bleaches orange. It takes a lot to strip the color, so tread lightly if you are prone to changing your mind re: your hair color. Henna is permanent.
The only way to get rid of it fully is to grow your hair out.
Hope that helps you make an informed decision!