A really quick answer (you can search online for the long, detailed answers - try
www.digido.com):
If you're bouncing a 24 bit file to keep working in 24 bit, don't use a dither plug. You only want to dither on your final master. Mastering engineers prefer getting 24 bit files so they can do the final dithering. 18 and 20 bit are rarely used in any current formats. Some ADATs run 20 bit. Only use one instance of dither, preferably POW-r or IDR.