Tire Pressure Monitoring Systems TPMS

I do apologize for raising the ire of some posters on this forum by re-asking an old question, but a portion of my request for information was passed over. I'm concerned about the attaching of the sensors to the wheel covers. It seems that a few of the people I have asked about the sensors had them attached to their wheel covers. Is this necessary?
I do want the pass/flow through type and am concerned about a weight/balance issue.
Thanks