I write a work order/ contract for virtually everything. I don't always get it signed. Most everybody I have worked for has been no problem. The biggest problem/ only time I didn't get paid was when I sub-contracted for another guy (which I ranted about, and used as a reminder for people to get it in writing).
For people on the up and up, it just serves as a tool in case one side or the other forgot we included/ didn't include something, and the price.
One guy I work with at Parks thought we agreed on $650 when it was $675. We both knew what the scope of the work was. I just put 675 in my phone contacts next to his name, so I had a reminder. Came in handy. He didn't have any problem with the $25, and felt like he was still getting a deal. I didn't have any problem saying, 'actually it was $675', and I didn't mind getting the full amount, as I was giving him a good deal.
Never had to chase anybody down for money, thankfully, but have had to refer to scope of work a time or three.
I'll scan my contract sometime, in case it is of use to anybody.
One thing that I have is a signature for Acceptance of Proposal, including that they own the trees or have the authorization to have the work done (if its a neighbor's tree I like to have a simple note/ email from the neighbor with authorization to do the work and have access).
Then, I have a signature and date area for Acceptance of Satisfactory Completion. I feel that this will avoid some liability if someone trips over a pine cone and wants to sue, or there is a divot and they twist an ankle the next summer mowing the lawn. Can avoid a call back if one owner okays the clean-up but another wants something more that would necessitate a return trip expense (especially if I do all the negotiation with the husband or wife, and then the other "thought" we were going to do something different).