2003 Cadillac Seville Is it ok to replace only one strut?

I started getting a squeak in the front end and took the car in for service. The problem was diagnosed as a defective front strut and they replaced it, but only the defective one. I always thought that struts and shocks need to be replaced in pairs, but the dealer said that was not true. So, my question is "Is it ok to replace only one strut? And, if not, what problem(s) could arise?

thanks
Frank Progl