I water wherever my ipad drops me too. Sometimes, though i'll take the time to find mature crops or trees.
I had one neighbour who specifically requested that her neighbours water neatly and got upset whenever that request wasn't followed. Well...I made sure every time i went to her farm to water in a haphazard manner. Lol. I actually took pleasure in doing that, i am most ashamed to say. She deleted me. I laughed. I still pop in every now and then and water all over the place
As for collectibles: i find that vegetables guve me the highest collectible drop rate. I tend to get the rare ones whenever i water the veggies. Sometimes i'll even get two rarer ones in a row. I must try watering only trees for a day and see what happens.