In my opinion, it depends on your purpose. If you have time and want to update your Java knowledge also, you can go to the Sun's website, compare SJCP 6 and 5, try to learn new objectives. If you think that it's enough, try to practice with the tests as much as possible to have a good preparation for your test.
If you are really confident on your preparation for SCJP 5, then you can add on to go ahead and add knowledge for SCJP 6. It's worth it with the fact that there are only a few more topics added. Precisely Navigable HashMap and sets and console api's and few other system utils.
Unfortunately there is no substatial material for SCJP 6 for now. But, you can search this forum to have some good information.
Also, I'll like to inform that passing percentage for SCJP has been increased to 65%.
So think well, choose your options, and go ahead with SCJP 6 if you are confident enough.