Keep the spackle and paint handy for when you are done. If you are very insecure about where to put things, try putting masking tape on the wall around the border of the object you want to place there and see how it looks. You can also use large pieces of paper to get a feel for relationships of items next to one another. You can also do it on the floor first. Leave some open space in your room.
Every room needs a place to breathe.
Finally, remember that what you are doing isn't permanent. Live with it a few months, and then if you don't like it, you can always change it.