I doubt if there is a homebrew CW rig that couldn't be improved with some extra filtering.  If you have an extra opamp on your board - or room for one - here is a great notch filter that will improve your audio filtering.

Most of the interference that is noted in a CW receiver will show up on the high side of the rig's filter center frequency.  Adding this 1500 Hz notch will cause a faster roll-off of those interfering signals.

Many super-narrow filters have a reputation for ringing, and blurring the transitions of the CW characters.  This has to due with the group delay of the filter not being flat trough the frequencies of interest.  This filter, on the other hand, has a flat group delay, and performs the notch filtering function without distorting the CW characters.  You may scale the component values proportionately to move the frequency to where you wish.

Schematic: