Just as it is for all earlier Mac Pro models, upgrading the memory in the "Mid-2010" Mac Pro is quite simple.
However, it is critical that memory meet precise standards to function properly. In the PDF instruction manual, Apple specifies that all UDIMMs must meet these specifications:
PC3-10600E, 1333 MHz, DDR3 SDRAM UDIMMs
Error-correcting code (ECC)
72-bit wide, 240-pin ECC modules
36 ICs maximum per ECC UDIMM