Depends on what they are haggling for.
If somebody just texts me saying can you do (insert service here) for 20 quid. I would definitely ignore them. Timewaster alert!
If it someone I have seen before and I have availability then I would consider it, if it just knocking a tenner of my rate
It tends to be, if I think its a guaranteed booking from someone I know, I wouldn't treat it as rude, as I know guys spend a lot of money.
However, I would much rather they didn't try and haggle! But I guess that's not unusual