I can think of no downside to washing your pellets other than the time it takes.
As to lubing them, that may or may not be beneficial depending on the substance used for "lubing" , the pellet, your barrel, etc, etc, etc, etc.
Lubing is one of those areas where you actually have to try it to see if it helps you and your particular combination of pellet, gun, etc.
For me, I have found that lubing *can* help with accuracy, but not always. It *may* help to keep leading the barrel to a minimum, but I can't say that for sure.
Give washing and lubing a try and see for yourself with your own equipment. That is the only way to know for sure. (smile)