The more you can limit or remove vegetation from your structure the better. Pests use vegetation as a highway to and from your home. Pests sometimes make homes in exterior vegetation and move from those nests into your home later.
Many types of pests get their food from insects that live on vegetation - so cutting back makes them go further for food. It also makes it easier to see if you have pests and easier to control. If rats can get to your roof - they will get in. Vegetation is a major access route to your roof and from there - inside your home.
Vegetation against your structure traps moisture against your structure that makes it rot faster, think very expensive repair bills, and moisture in your structure is a huge draw for many types of pests (Carpenter ants, moisture ants, termites, etc....
We recommend trees, shrubs, and vines be maintained at least 3 feet from roof, gutter, and eave. Grass is kept trimmed back from siding and decks (consider installing gravel beds), and vegetation is trimmed back and thinned out from around your siding allowing for air flow.
See more at pest prevention