Nope attitude. The picture will always be the same with the engine not working just before the critical angle of attack. You can't see angle of attack you can only see attitude. The resultant airspeed will change with weight (but very little in a GA machine). Works on big things as well when you get 15-20knts across the operational weight band for the approach speeds. Only screws up if you have a increased Vref due to Vcma issues.
Which is why some of us teach a series of pictures for the different stages of flight. Once the pilot has them in there heads they will never get near stall and the ASI is just a bonus to confirm what they know already. Takes quite a bit off the work load if the pilot can just pitch for the best glide speed attitude and then put the ruff required amount of trim in instead of having to focus on the ASI and chase the needle until they get it right. Do you really think instructors look over at the ASI to get the best glide speed when we are demonstrating?
PPL student with all the instruments covered can do a spot on PFL at exactly the right speed in my experence, as long as they have been taught the basics properly.
Again it comes down to the first lessons building the foundation for everything else. Rushed into the circuit or not given enough time to get them understood at the fundemental level and it is with the pilot for life.