We did not give any tips. But I think it also depends on whether you pay a general contractor a lot of money, or if Uncle Karl and his Skat buddies tile the bathroom for you. If you get favor prices or free help, I think tips or small gifts are appropriate. I once photographed a wedding free of charge and received a small gift basket and a 30€ voucher for the swimming pool, which I thought was a good idea. Otherwise, on the (GU) construction site, we always have water available for everyone, and now and then we bring a crate full of chocolate (Snickers, Knoppers, Duplo and stuff like that, and a few candies).