Happy customers are one of your best sales tools, so you want to keep your customers happy. This means, among other things, maintaining great communications with them. One element of this communication is talking about things like endorsements.
Look at things from your customers' point of view. Their name is part of their brand, which they are proud of just like you are of yours. They want to control and protect usage of their brand. If you want to use their name -- their brand -- in endorsements, it's proper to seek permission. When they grant permission, then both of you are happy. You'll be happy to have a great endorsement, and they'll be happy to publicize the fact that they found a great product.
Standard practice in all the ventures I've been in has been to spell out endorsement and publicity policies in our customer contracts. This way the customers know that you'd like to do this. Some customers won't want you to use their name, not because they're unhappy, but because that's their own policy. You should respect their wishes if that's what they desire.
So, in short, tell customers you'd like to do this. Some will be flattered and happily allow you to do this. Others will not want you to use their name, and you should follow their wishes. And no matter what, put it in writing.
Best of luck!