Carpenter bees make their own holes while Mason bees take advantage of preexisting holes. At this time of year I would expect them to be in the final instar of the larval stage or have had a cocoon for a few weeks. The early spring heat in March is the last thing they need to fully develop. There's about a 2 week period where they begin opening emerging and from there they're only around for 4 to 6 weeks of the year. However, there are a few species at work here such as Leaf Cutter bees and Mason Wasps which are more common over the summer. Keeping a few free nesting blocks around all year with varied sized holes can help attract different species.
I know Leaf Cutter bees will use the leaves to cherry trees and sunflower pedals, but I don't know if they're very specific.
At any rate, Mason bees, and some mason wasp, are great pollinators. They only travel about half a mile from the tube they're working and aren't really picky about the flowers they visit, similar to bumblebees. The trouble is your resident Mason bees might not be active when your garden is blooming. And though they can get deeper in most flower they don't compare to the numbers a Honey Bee hive can offer. So they might do a better job with a few blooms on a tree, the over all result won't compare to Honey Bees unless you have a great deal of them around. I read once that 1 female mason bee can provide for 20 to 30 young. A number of these are males that die after mating with the females and there are parasitic mason wasps/bees that lay their eggs in place of host Mason bee.
As far as beekeeping goes though these are the easiest bees out there. They don't sting or rarely do anyhow, they're not aggressive. You just drill holes in dead wood (untreated preferably, and not all the way though the wood) and they show up.
What size holes does everyone have the most success with? I know most mason bees are tiny but the imported Resin Bee is quite big, a bit smaller the Large Carpenter Bees it uses the old nests of.