I believe that starting this year, any prematch offer is illegal (or break the match rule). In the years past, it has always been on the no-no side but this year, NRMP appears to be determined to crack it down and looks to be more serious than ever.
However, I would not be surprised if programs still do some prematch offers, along with we will rank you here and there, so be sure you rank us high.
I am not familiar with all these prematch etiquettes. I have only gotten 2 interest letters out of 11 programs I have interviewed at. both letters were more on the line of "we are great, consider ranking us" propaganda.
I might wait for another couple weeks before contacting programs and gauging interests.