If your application is calm, the URLs that the rails give you are SEO-friendly by default.
In your example, customers/1234 will probably return something like
<h1>Customer</h1> <p><strong>Name:</strong> John Smith</p> etc etc
Any current SEO spider will be smart enough to analyze the landing page and extract "John Smith" from it.
So, in this sense, customers/1234 already a โgoodโ URL (unlike other systems in which the client would have something like resource/123123/1234 for client 1234 resource/23232/321 for client 321).
Now, if you want your users to regularly use URLs (for example, in delicious, etc.), you can start using logins and reading fields instead of identifiers.
But for SEO, identifiers are just perfect.
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