Great idea. Although it's pretty easy to remember:
EST: GMT -5
CST: GMT -6
MST: GMT -7
PST: GMT -8
The only real variances come in to play with daylight savings time. Although some countries do follow DST, not all do. For that matter, nor do all states. For instance, I believe that Great Britain follows DST, but not necessarily on the same dates as us. Furthermore, I believe that officially GMT doesn't change, while regions in GB that are in GMT, officially become GMT+1. In which case, we're still the same number of hours behind, say, London when us and them are on DST, but we're not the same number of hours behind, say... Ghana, assuming they don't follow DST (I have no idea about Ghana, but they're approximately the same longitude as England.