I cannot stress enough the importance of proper design and testing of your plc/slc program as well as proper documentation.
when our company first started using plc the program writers would just list inputs and outputs in the program and nothing else
this was because when there was a problem they would be called in to correct it (charging outrageous fees as well)
now most because the early plc's had memory constraints if they did not have memory modules to upgrade with

todays plc's/slc's have a larger memory module and the text doesnt affect the operation as the text is not uploaded into the plc/slc
designing the program on paper with notes makes it a lot easier when you go to write the program in the laptop or pc.
observing the operation of the equipment shows you the end result of the program and allows you to see where improvements can be made.
keeping in mind that the first and foremost consideration is safety

no operation must ever be undertaken without safety being the first consideration and there is absolutely no exception to this rule
would you want to be responsible for someone getting injured or killed because of something you overlooked or ignored?
I know i wouldn't

write, test, and document every rung of your ladder program

if you have to build a simulator to test the program before ever applying it to a piece of equipment people are working with or around