Please read the following article about the importance of running debug="false"
Excerpt:
[Keeping the debug="true"] causes a number of non-optimal things to happen including:
1) The compilation of ASP.NET pages takes longer (since some batch optimizations are disabled)
2) Code can execute slower (since some additional debug paths are enabled)
3) Much more memory is used within the application at runtime
4) Scripts and images downloaded from the WebResources.axd handler are not cached
After freefaller's remark i'd like to add that the configuration value is not important at the time of publishing, only at the time of running. So you could compile your assembly in release mode with debug="true" and deploy it with debug="false" without any problem. The parameter only acts on runtime configuration