My cover letters are always customized.
I have a basic 5-paragraph letter which I polish and trim. I use the job announcement as a guide for the cover letter and try and include some relevant key words and emphasize some of the traits they are seeking in the job description.
I also always mention the name of the company in the cover letter so they know it was written specifically for them. For example:
I am certain that I can bring the professionalism, the energy, and the results a company like JPMorgan Chase demands.
If including a cover letter and personalizing it adds a few more horsepower to your application, it is insane not to do it. Just my opinion.