Its really up to you. I have a friend who is an expert mechanic and, if the machine is working, he favours only an oily rag. I do not like to see rusty or oily elements so decide to remove/treat the rust etc. and paint them. However once I remove certain parts I find others that would benefit from the same treatment and invariably end up totally dismantling the machine and repainting it. I find that if I only repaint the 'tinwork' the remainder looks even worse and gives the overall impression of being half a job. I am not endeavouring to reach 'show standard' but just to ensure that I remove or stabilise any corrosion and paint so that I should not have to revisit it.