It sounds like you are getting it to work correctly sometimes, and others you are having troubles. So I just did a test and was able to get all of my snapping correct in the scene and the viewport. What you want to make sure to never do, is move any object at all, with snapping to the grid disabled. This will offset your object, and then when you try to snap it to the grid again it will be wrong.
I generally tend to use the values of 5, 10, or 50 when snapping as those cover a nice range of close and far distances when dragging objects. Below is an example of the small blueprint I created using the modular assets we have in the engine content. Keep in mind also, they are all pieced together using the same dimension walls (400x300) which is why I was able to get them to piece together cleanly and symmetrically.
Modular Blueprint Walls and Door
In Scene
As you can see I have maintained the grid snapping size, but you will also want to take note of where your root is for your blueprint. If you want the root to be centered on the object, you can create a default root component (0,0,0) and then parent your modular assets to this root.
Hopefully you can get proper results with a little practice and further experimenting. When working with modular assets, you want to keep in mind the size and dimensions of the objects you are piecing together, as even though they might snap to the grid, they will be offset due to their own relative scale.
Cheers,