For the tack, you want a low stretch line because there is a lot force pulling it away from the mast
. If the reefing line sretches and doesn't take the load, the slides closest to the reefing cringle will and they aren't designed for that. For the clew, any old polyester line will do as a bit of stretch is actually desirable to ease loads in gusts.
On my two line reefing reefing system with three reef points, am switching over to Dyneema
cored, polyester jacketed line about 5/16" OD diameter for the tack. It is really easy to haul down the luff by hand and get the cringle snug enough against the mast
. Have 7/16" StaSet clew reefing lines though think if I was to do it again would use 3/8" StaSet. The loads aren't that great so don't need that heavy a line dangling off the leach of the sail. The reefing lines are quite long 23' to 45' for the tack and 39' to 62' for the clew by the time they get back to the cabin