Just had my small roof and gutters/downspouts done in one day last month.
I put out a cooler with ice packs and a 12 pack of bottled water and slices of fresh watermelon in ziploc bags (enough for everyone to have 2-3 pieces). I also let the roof crew know they could use my bathroom - just knock. It was all gone by noon when they finished (the day was HOT and they worked 6am-12pm), and the roof looks wonderful.
Afternoon gutter and downspout crew (here from 1pm-2:30pm) each got a bottle of water and a bag of m&ms (they had brought a kid, since it was a short job, so I figured the kid would rather have candy...and I was right). Same news on the bathroom. They also did an amazing job.
I did not tip money - they didn't even really stay to be tipped. They just cleaned up and waved themselves out.
I was very happy with the work and company. I don't think anyone expected or planned to be tipped, but they were very thankful for water, a snack, and allowed bathroom use.