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.

Schematic: