Do RAM ranks matter in the 2013 Mac Pro?

Yes, they must match. The 2013 Mac Pro requires all four slots to use the same rank configuration—either all single-rank or all dual-rank. Mixing ranks can cause boot failures or throttled performance. Also, use only registered ECC (RDIMM) at 1866 MHz. The easiest path is to buy a known-compatible kit from a vendor like OWC.