Do they need regular teeth whitening? Sure, if that's their image
According to one article:
A staggering 45 million Americans – 14 per cent of the population – have had professional teeth whitening, according to data guru Mintel. This is compared to just 3 per cent in the UK.
It appears that Americans value a white smile very highly:
...half of Americans believe a bright white smile is the most memorable feature when first meeting someone, according to the American Association of Cosmetic Dentists (AACD), more important even than what that person says.
If you are an actor looking to 'break' America, you may consider having a "desirable" smile to enhance your image.
While many roles might require false teeth which enhance teeth in some way (for example Jim Carrey in The Mask or Rami Malek in Bohemian Rhapsody), an actor may well want to maintain a certain image off-camera if they felt it might lead to better public image, or perhaps more regular roles.
In a similar way, some actors (male and female) will wear makeup to red-carpet appearances - so it's clear that there is an expectation either from themselves, or from others (perhaps management) to maintain a "perfect image".