As a rule, we really prefer that students do their best to stick to the instructions as they're written - while not everything is a matter of life and death, opening the doors to students modifying things willy-nilly is asking for trouble down the line (and it would be rather demanding to have students then try and confirm whether their modifications are okay, instead of just sticking to what we instruct). As to your question though, performing the error analysis for a full page leans into identifying patterns of behaviour, whereas analyzing each box one at a time will make you more likely to put too much weight into issues that may be one-offs in the grand scheme of things.