I leave the jacks slightly loose, but always in position, and in 14 years and about 20K miles have never noticed any chafe on the mainsail. Mine is a simple arrangement: the lower line is attached in two places to the boom forming an upside-down semi-V shape. A topping lift on either side holds them up (the mast was made such that these are internal halyards which helps keep things clean and fewer lines to bang on the mast which I hate as much as Brian does). I have full battens and the jacks keep the battens aligned over the boom, which is important both when furling and when hoisting. It is a semi PITA when a fully battened mainsail falls off the boom and onto the deck. You ony have to worry about the top batten catching the jack lines, once you get past that you are home free. I used to have little blocks where the two lines meet, but one broke and I simply tied a bowline to make an eye in the upper line and it works fine.

However you do it jacklines are well worth it.