Telemetry is what tells you if it changes, from the minute you put it on until the minute you take it off. If you are looking for more subtle changes of ischemia, as opposed to merely dysrhythmias, you should be getting 12-leads anyway.
That said, most people who buy themselves a slot on telemetry have had a 12-lead anyway sometime recently. If someone has been in the hospital admitted thru the ER or OR and is now on tele, I would find it very unusual for a 12-lead not to have been done at some point. For most adults a 12-lead within 30 days is standard for preop clearance. Doing another one merely because you are about to slap on tele leads sounds like padding the bill to me.