If you intend to sell online via your printer, or use their retails channels, you will almost certainly need one. I can't remember if it's true of Createspace, but Lulu requires any book available for sale to have an ISBN. This leaves you with the choice of paying for your own, or using the free one the printer provides.
ISBNs can be expensive, depending on where you're based. At time of posting, a single ISBN costs around $125 in the US, although apparently Canadians can get them for free. The only advantage I can see you getting your own ISBN is it allows you to be listed as the publisher in the ISBN database (otherwise it's listed as Createspace, Lulu, etc). If you're not publishing your books as a small press this may not matter to you.
Each new edition of your book will need its own ISBN. This is the case whether you're updating the content, or publishing it on more than one POD site. If you plan on tweaking your content to keep up with a topic, it could get expensive if you're buying your own.
Once you have your ISBN it needs to go on the copyright page in your front matter.
Next: choosing your price.