Do you charge more if additional person(s) move in?

[Alison P.]

I would appreciate any comments or experience.  My lease states that if someone additional lives in the property for more than 48 hrs, the tenant must get written permission from the landlord.  I drive by my rentals several times a week to keep an eye on them and noticed that an additional vehicle was parked at one for over a week.  I contacted the tenant and asked if anyone else had moved in and that he needed my written permission if this was the case.  He confirmed that he had moved in a woman and her child.  I got their details and I'm okay with it because the lady has tidied the place up much nicer than the single guy was keeping it.  

When I mentioned what had happened to a landlord friend of mine, she told me they have it stipulated in their lease that if an additional person moves in, there will be an additional amount of $100 per adult and $50 per child per month added to the rent.  This would certainly make tenants think twice before sneakily moving in a new girlfriend etc.

What are your thoughts?  Do any of you do this? Do you think it's a good idea, bad idea?  Would love to hear your opinions. Thanks!