Only if you can't clear a clog with one or two cleaning cycles and one deep cleaning cycle. There are several methods noted in the first item under the FAQ section of this forum (see links at the top) to consider before buying a cleaning cart. If you already have the cleaning cart it can't hurt to use it if the clog doesn't clear after the software cleaning cycles.
I never had any cleaning cartridges. Never used one for my 8500. I can't see a need to use one. Why do you think you need it? Just use the printer regularly. It cleans itself by using it. I can think of situations where you may need some user invoked cleanings. If your 8500 prints a few of the colors much more than other colors. Then you may consider those rarely used colors more potential to clog up. Just a thought. The printer does its own cleaning controlled by the printer firmware. You probably do not need to worry about that. Just use quality ink and ink cartridges. That's the best bet. Use the printer regularly. Yep, don't try to fix a problem that doesn't exist.
I don't have any problems with my printer, but when I was buying some more cartridges yesterday I noticed the cleaning cartridges which weren't there before. I did pass on them since I didn't think I needed them, but just wanted to double check.
I did make cleaning cartridges for an Epson printer I owned and used instructions from Arthur Entlich, a participant in the comp.periphs.printers newsgroup, to clear a clogged printhead. Since the Canon printhead is removable, on the rare occasion when you might get a clog that won't clear with the canon cleaning routine, you can remove it and carefully clean it with one of several techniques. The best cleaning technique, however, is to print frequently!