Paint straight line at ceiling like pro without tape. Paint ceiling first, wrapping corner. Take ubiquitous back edge and gently score mark. Just use corner as naturally flowing guide. Using brush loaded with paint, just cover line.
DIY: How to Install IKEA Bookcases So They Look Like Built-In's - lots of pictures and info on how this project was done, including how to allow for electrical outlets and mouldings - via Storefront Life