There is an old saying among attorney's; "A verbal contract is only as good as the paper it's written on."
If you and the rental agent agree on something you want done, or something you want to do yourself, or a repair you need, or a rent or deposit reduction, get it in writing. Make sure that you have a copy and the rental agent has a copy and that both of you have signed it. Then, hold on to it. It may be the most useful thing you have later on.
After it is signed, it states exactly what the two parties are bound to do or not do. There is nothing legal about an "understanding" between you and the landlord. If you have an understanding, you should have it in writing (see Rule One).
A common problem in this area is when someone signs a lease for 10 months and then drops out or moves after one semester. If you've signed a 10 month lease, you are responsible for the rent money on that property for 10 months whether you live there or not.
The same thing goes for roommates. Often two people will sign a lease and discover, after two months, they can't stand to live together. If you change roommates, get them to the landlord immediately and change the names on the lease. If in doubt, see Rule One. No one wants to pay the rent on someplace they aren't living.
Lessors (landlord/managers) are not looking for trouble. They want to have good tenants and for their tenants to enjoy living in their properties. From that, they get good referrals and more good renters and you are guaranteed a good referral when you leave.
If you are a good renter, they will be happy to tell others.