For the most part, I feel that there really isn't much need in lettering the truck and/or trailer, unless you are wanting it to look even more professional. When you are out working, people see your truck and your trailer, and most often you on a mower! They know what you do, and where you are working at that given time. If they want an estimate, and they live in the area, they can stop and talk to you ... or ask for you card.
This isn't rocket science .. it is lawncare (for most of us on here). I see trucks and trailers with mowers all day long ... everywhere. And we know what it is they do.
Now if you want to do it for the image ... that is totally fine too. To me ... it's a personal choice ... and not something you "need" to do.
(and the magnetic signs work great)