sir av_phile, i've read somewhere that,for example if you're surround is placed beside your listening position.It's better if you will set the delay/distance 25ft? or less? 
....im confused... how about the distance of L,C,R...thanks sir!
Let me out your question into proper perspective based on my limited understanding.
The audio engineers who mixed a 5.1 did so using monitor speakers that are of equal distances to their position. Upon playback, it is not only ideal, but a MUST that all the 5 speakers around you are at equal distances from your listening sweet spot. You can picture an imaginary circle with you in the center and the speakers are located along the circle path pointing at or facing you. If your in such a position, then you can forget about setting the speaker delays.
However, in the real world, it's possible that such an imaginary circle positioning of speakers or your center location cannot be achieved.. So THX recommended delay or distance compensation computers in AVRs.
Channel delays operate in the digital domain to COMPENSATE for a situation where your listening position does not yield equal distances between you and each speaker. So you must compensate this by FOOLING the AVR into thinking all the speakers are equidistant to you. You do this by increasing the delay in a speaker that is closer to you than the rest, or decreasing the delay in a speaker that is farthest from you, relative to the rest.
AVRs with milliseconds adjustment is more direct. There is no recommended milliseconds as far as I know. You will have to do some experimentation as each room and speaker placement is unique relative to the listener position. If one speaker is farther from you than the rest, simply reduce the delay for that channel. And vice-versa. You can start with all the channels having the same delay milliseconds and start reducing the delay of one channel with farthest speaker or increasing one that is closest.
AVRs using distance in ft to set the delay uses a distance compensating algorhythm. You set the ACTUAL distances between you and each speaker and the circuitry will compute to make the sound arrive at you as if all the speakers were equidistant to you. I personally think this method is the most user-friendly, acccurate and simplest.
So to your question, I don't see the merits of setting a false distance to the surrounds, unless you like what you hear. Nothing will prevent you from setting false distances to any of the speaker. The AVR will still compute to compensate to make it appear all speakers are equidistant and the delay adjusted automatically for each channel. If you like what you hear, then I see no problem.
