I didn't tip for either installation I had. Maybe I should have. I am pretty clueless when it comes to such things.
But I would not tip before hand. My first installer was terrible, and would not have been any better if he had been given some money up front. The second one was great, and could not have been any better if I had given him money. If you want to get better service, be very polite and respectful. People want to be nice to people who are nice to them. And don't call the place I called the first time. Bunch of jerks, they were.
I don't believe they are allowed to accept a tip but I went ahead and tipped our guy. I don't see in tipping if they work directly for E*, but if a contractor I would depending on the result. My guy did a great job with very good attention to detail. I showed him where to install and we bounced questions off each other to make sure what I like before drilling holes where ever and to make sure it works for him too. The guys who don't work for E* could use the extra cash I believe because it doesn't sound like they get compensated too well.
I tipped my guy when he was leaving because he was there for a long time and I feel he did a great job and went beyond what a normal install would cover. He climbed in my attic, fished the cables down and was great.
I always tip. The amount depends on how hard and how long the job is. I don't tip before hand though. I offer them sodas or drinks during the installation. I tip before they leave and ask for a phone number I could reach them at if I have any problem with it.