Why risk it?
Another poster (MikeV) recently refurbed his MB head as well and used the old bolts. Guess what? He's getting antifreeze in his oil.
Yes they do stretch as I've heard and if it were me I'd never risk having to take apart my engine again because I didn't spring for a set of $1.50 head bolts.
Good luck in whatever you decide~
'91 300E, 206k miles