Very good post and makes good sense. I did what you are talking about just recently with an irrigation company. Some yards I do don't have irrigation. When they ask me I always give them a name of a company I used at a house I used to own because I know they offer good service and do a good job. I ended up getting some free work done at my house because a referral turned into a job for the company.
I also always try to give a wave or nod to any other landscape workers I see working or driving by.