The OE brake paste is about 75 cents a packet, not to mention most places will give it to you free with the rest of the parts. Why mess with something so inexpensive and important?
The reason you want to cover the fluid reservoir is so that any brake fluid that splashes up goes no further. While the cap normally performs that function you do not want the system fully pressurized. It makes prying back the pistons much easier. By the way, if ANY brake fluid contacts any painted surface the paint is toast!
Just carefully follow the steps we outlined in the DIY article and you should be fine.