Doing this for a second or two isn't going to do any permanent damage, but going to a live show and standing on either side in front of the mains for 45 minutes - 3 hours will. Direct exposure @ 120 dB takes a little more than 3 minutes to cause hearing damage, so taking a few seconds to put your head in front of the cab isn't going to end your career as an engineer, but playing a show, or having band rehearsal without earplugs will. 120 dB is really, really, really loud.
This is something I picked up from Slipperman, but YMMV.
Yeah its loud, so have the bass player do it. He doesn't need to hear high frequencies anyway, right?