If you don't, you had better plan on big problems someday.
I did a job for a prominent advertising company a few years back. They had trees growing up on the highway right of way that needed to be eliminated to improve the view of their billboard. The work was on a huge hill, and was rather inaccessible from the highway side of the work.
We knocked on a lady's door, and asked her if we would be able to use her yard and driveway for access. We explained that we were clearing brush from the highway side of the fence, and that we would be happy to do some clearing/trimming on her side of the fence to compensate her for any inconvenience. She was happy with that plan, and we agreed to eliminate a bunch of weed trees in her yard and to prune the trees back from her roof. All seemed perfect.
When we cleared the highway property, she realized that her "privacy" had been diminished. When she found out that we were working for the billboard company instead of the highway department, she hired a scum sucking bottom feeding lawyer. She denied ever speaking to us and sued for around $100,000.00.
She claimed about $2000 for each little weed tree eliminated, and they even were claiming each weed tree in her yard that was on the highway right of way. Fortunately, there was a marker post in her yard that clearly said where that line was. Honestly! The scum sucking lawyer had "experts" that counted each little 1/2" weed tree stump in her yard AND on the DOT right-of-way. Then the billboard company had to hire their own expert to refute the value of the trees and the damaged plant count.
The billboard company expected me to cover the claim, despite the fact that I had written exactly what I was going to do in the contract they signed. In the final outcome, they and my insurance company settled for somewhere around $50,000.00, and my insurance company paid about 1/2 of the bill.
I told everybody involved to not protect me, let her sue me, and I will see her in court. I had witnesses, too. No matter...it was out of my hands. The billboard company was really mad at me, as though I didn't specifically tell them in writing what my plan was. They still call me for estimates on big projects, then they go do it themselves anyway. I only get the really foul jobs that their guys feel unqualified for.
So...you had better put it in writing if you are going onto the neighbors property! Quite frankly, I would recommend before and after photo's too, just in case.