Plaster Walls - Touch up or Remove and Drywall

[Mark Koontz]

I come across and have bought several early 1900's homes which typically have plaster walls - sometimes not in great shape. 

With my main focus being LTR, it doesn't raise the rent significantly by drywalling the entire house. I'm more thinking about the resale. 

What does everyone do with old plaster walls? Just touch up where needed and paint? 1/4" drywall over top? Tear out and drywall?