Changing the engine oil and filter is about as basic a DIY operation as there is, but there is always a lot of discussion about how to do it properly; such as, should the filler cap be removed before the drain plug is removed, etc. I came across the below "How to Replace The Engine Oil and Filter" on Toyota's official website and not having seen it on PriusChat I thought I would share it for what it is worth. Note that Step 4 of the below instruction does say to remove the oil filler cap before removing the drain plug. http://www.toyota.com/toyota-owners-online-theme/pdf/EngOil_Filter.pdf
Toyota's remiss in not including basic maintenance in the Owners Manual, IMO. Regarding when to remove fill cap: I find it makes little/no difference. Thanks for the link
Whether you remove the cap or not, common sense dictates that if there is only one way in and one way out, be sure that you will be able to get the oil/fluid in before you take it out. Same as locking yourself out of your house. Its not a big deal but could cost you more (time/money) than you had planned.