OK.  Here’s the deal.  Probably 8 or 9 months or so ago, one of my friends from up the street mentioned that she had the plumber in and through the course of the conversation, she mentioned having given him a tip.  No, smarty pants..not the stock market.  A money tip.  I thought "A TIP?  Am I that far behind the curve in manners or is it something I’ve been missing all along?"  I didn’t question her as I felt faintly embarrassed since I never have tipped a plumber.  I have to practically take a loan out when the plumber is coming over. 
Then, the day before yesterday the plumber was here to install a faucet for the outside  hose and to inspect and "pronounce" over the kitchen faucet set.  As we are long time acquaintances and get along great, he stood there talking a good 45 minutes after the job was done.  In the course of the conversation (very enjoyable..I like talking to Scott) he mentioned how his helper got a bigger tip (monetary) than Scott ever had.
I was rocked by this.  So my question is:  Is it customary to give plumbers tips?  Serious question.  Please let me know.  I can’t afford a tip adding 18% more onto an already very high price. What’s your idea on this?