Do I need permission from my customers to post photos on my website of projects I've completed at their homes?
Generally, if you are using photos for a commercial purpose (which a website tends to be) you need to be mindful of whether the photos would be deemed "an invasion of privacy"--something that varies with state law.
The fast and easy way to deal with the situation is to secure permission from your customer. If you've done a good job, they likely won't hesitate to allow your use of the photos. Make sure to get the permission in writing. A qualified attorney can create a form of release for you to use with your projects so that you can develop your portfolio.
If you use the photos without permission, the waters get muddied. If the customers were unhappy, they may not want their house featured because that could be deemed some kind of endorsement of your work. If there are any unusual features to the house, they may be recognizable despite the lack of customer names.
It's best to consult with an attorney so that you're clear on the parameters of the law in your state.