Use the default settings on the indicator in my option. Always, always, always use default unless there is a compelling reason to do otherwise. If you use default, chances are you're looking at what other traders with similar strategies are looking at.
Besides that, numbers that make sense are good to use. For example, 12 5 minute bars makes 1 hour, so you could use a 12 period RSI instead of the default 14.
Recap:
1) Use default settings
2) Only change them if the number makes sense
