for sub-breakthrough travels, i always close my eyes.  in fact, i like this special eye covering called a "mind fold" which has cutouts for your eyes so even though they're covered, one can keep eyes wide open in pure darkness. 

earbuds and mindfold for sub-breakthrough always...i have a special playlist just for these travels...

for breakthrough, i find it really doesn't make any difference.  in fact, upon returning i am never really sure if they were open or closed.