Don't worry about the Grid either...
The world is not built around 5 foot squares. In fact homes in the US are built around 4 foot frames, not 5 foot. Let's say you're creating a 12' by 18' cabin. Are you saying you can't place a 1 square = 5 feet grid, because the walls on two of the sides land in the middle of a grid square?
I say don't worry about it. The grid is an artificial tool to help your players understand the room they have to work in. Walls shouldn't have to sit on the edge of a grid square. Create your maps at the scale you want. Then place a grid to help size up things for players. Don't build structures based on a 5 foot grid, that's completely silly. (If you want to, that's OK, but not necessary...)
If you look at any of my maps with square grids, walls and stairs and hallways often land in the middle of the squares. As long as your players can figure it out, its no problem.
Consider the fact that you have a 10' wide hallway, and rooms on either side are 10' x 20. What are you going to do about the walls between them - are you forced to make 5' wide walls, or worse 0' wide walls (lines) because the walls will take away space from the rooms or halls?
The grid is a guide for players, it is not a standard to build around.