I'm using an RB751G-2HnD without an external antenna connected.
Is it dangerous?
this is true, what Uldis saidThat's not a problem, but don't change the tx-power to higher that the default card rates. Uldis once said on a MUM session that tx-power is only used to decrease the value and not to increase it. What you certainly should be careful with is tx-chains setup and antenna mode in the 'HT' tab. That particular topic is very long, but it covers your question in much detail:
http://forum.mikrotik.com/viewtopic.php ... na#p300771