I have had two shower valves (one approximately 45 years old, one approximately 20 years old) malfunction. We were able to repair both by removing the cover and replacing internal components without removing the shower valve or accessing it from a different room. I believe most modern shower valves can be repaired rather than replaced in most situations. Personally I would not make an access panel if it visually detracts from the look you want in the laundry room. It is unlikely you will ever need to access the shower valve and if you absolutely had to, you could replace the backsplash without nearly as much headache as redoing an entire shower where waterproofing is involved.