Clients have the final say. Even if you disagree with it.
Disregarding that there maybe technical reasons why uppercase is a bad idea. And disregarding the fact that upper case lowers readability and often has the opposite effect of "emphasis"... Client direction is always the deciding factor.
If the client doesn't like it, don't do it.
If you are designing something and the client asks you to change an aspect, then by all means explain why you think they way you have it is better in your opinion. However, if the client still wants things change, then change them. The client is paying you for services, therefore they have the final word.