I have recently acquired a block of mason bee tubes that are partially capped with mud and some of the holes are capped with leaf material. The person that I got them from said that the holes capped with leafy material are occupied by "leaf cutters" and also do pollinating but come out later in the season. Is there anything undesirable about these bees?

They are not just OK, they are super pollinators. You are lucky to have them. Check out an earlier post on them. The small amount of leaf used for nesting is no problem.
Bberry

Thank you for your reply. I assume that they could be beneficial for late blossoming fruit trees. Jim

Leafcutter bees are usually smaller than mason bees but will use the larger nesting holes for msaon bees. Leaf cutter bees start emerging in June. They need about 3 weeks of warm weather to develop from pupa to adults, and when they do emerge, temperatures have to be above 70F for flying and foraging. These are warm weather pollinators! There are many native leafcutter bees, although the most common is the Russian leafcutter bee which is used commercially for pollinating alfalfa.

How useful are these? What kind of fruit pollinates after June? Pineapple guava and........?
John S
PDX OR