As I’m sure you’re already aware, you can save a diagram as an image in several formats, including PNG but not TIFF.
The quality of the images can be controlled in the options (Tools – Options), “Diagram” tab.
The “Image Memory Limit” controls the amount of memory the image conversion process is allowed; increasing this should improve the quality of large diagrams.
“Scale Saved Bitmaps to” allows you to set a higher resolution for the images.
I haven’t checked whether these also affect the copy-to-clipboard function (in the Diagram menu or CTRL+B), which is the way I usually copy diagrams into documents.