Extension tubes on lens

[pbourke]

I've just reconstructed some small beads using a Canon5D MKIII and 100mm prime macro lens.
However, to maximise the size of the objects on the sensor I would like to use some extension tubes. A quick test seems to suggest this will take me from using about 1/5 of the sensor to over 1/2.
I can't test on the actual objects as they are in storage at the museum, but I wondered if there were any issues I might encounter? For example, the EXIF data won't reflect the presence of the extension tubes.