Part of being a service tech is making sure the unit will be dependable and pointing out potential issues to the customer. They are not paying me for what I can do, because lets face it in the scheme of things, swapping a hi limit is not that technically involved. They are paying me for what I know, my experience and my knowledge of not only how something works, but why it fails to work.
Additionally, how are the gas controls at all related to this conversation? Would I recommend replacing the gas valve in addition to a failed thermopile? Absolutely not. Would I suggest replacing the thermopile when changing the gas valve? Depending on age and condition, probably yes. Dependability is as important to customers as cost of repair. Why risk an unhappy situation over an inexpensive part? So long as the customer is made aware of the situation as you honestly see it and is given all their options, there is nothing wrong with suggesting additional parts in the name of dependability.
Good luck explaining to that customer why he/she has to pay you to change the intermittently failing thermostat that is causing the hi limit open after paying you to replace the hi limit in the first place. To the customer, the open hi limit is the same issue they already paid you to fix, why should they pay you to fix it again?
I am definitely not a parts changer. I leave in my wake many happy customers who will wait for me to become available rather then get the next available tech because I fix am willing to fix the unit rather then the problem in an honest and ethical manner.