Hi,
I have read wireless faq located at http://wiki.mikrotik.com/wiki/Manual:Wireless_FAQ, but FAQ doesn't mention changing values not up, but down. I have some interferences with USB Creative Sound Blaster card with 5.1 sound setup which is sitting on desk near RB751G or RB2011UAS-2HnD-IN. Strange pulsing sounds are comming from my speakers. I cannot separate devices enought to make it dissapear - so I thought I change tx-power on RB. Unfortunetely when I set card rates to 5 I still hear cracking sounds, the only way for me is to put all-rate-fixed to 5 to solve this problem.

And my question is: Is it safe to change all-rate-fixed down? Is there any possibility that this settings will cause damage to my RouterBoad?

You are not going to damage the board by turning it down but you will hurt your performance.

all rates fixed 5 is an OK setting. for close distances you don't need more.

EMI happen. especially if TRX near acoustic systems.
got similar issues with my radios, AC and headphones from 3 from 4 of my cellphones, during incoming calls, sms or BS service[same was done by SW TRX, CB[11m and 70cm] WT].