You can mitre them, but if the block is going to be part of a larger top I tend to piece the border and I piece the quilt border as well otherwise you get a lumpy bit of fabric in the mitred seam and you waste quite a lot of fabric. (so cut border into 4 straight strips and attach)
I only mitre my bindings where the extra fabric helps it hold its shape.
However, without pictures as I don't have any to hand, I can give a quick tut on them and how to have the fabric in them minimised. Please note this is not how to do a mitred corner in your binding.
Cut your fabric lengths without forgetting to add 4-6" extra in length to account for the mitre.
Lay your fabric strip out along the seam.
Continue your fabric past the edge by about 2" at both ends.
Sew the strip to your block starting and stopping about 1/4" away from each end. Backstitch each end.
Repeat this on all edges.
Your tails will be overlapping at this point.
Fold the block over diagonally (RS tog) and line up your two fabric tails on a corner (so left side and top for eg) with the fold running a 45 degree angle between them.
Ruler on 45 degree angle, extend this out past your tails and trace onto the border strips and pin.
Sew along the traced line, make sure you find your original stitch line and continue from it to ensure no holes! Don't forget to backstitch at both ends.
Trim the excess.
Press the seam.
Repeat with other 3 corners.
Press.
All done