Are there any objective reasons to prefer a prefix over a suffix for an environment-specific login?
I'd rather have the environment be the suffix assuming I had to pick one convention.  Otherwise, it's too difficult to navigate through lists of login names that all use the same first five characters.  

Of course, if given my choice, I would prefer to use neither the prefix nor the suffix.  I'd rather keep the same username in all environments and vary my password and TNS alias to identify which system I'm connecting to.