How much leeway to decorate should we give?

Maintenance

My houses are in good condition in inner city areas. I have generally been quite slow to give tenants permission to decorate ie nothing in first 6 months and then piecemeal by agreement. I am noticing that it is becoming more of an issue. More tenants expect more freedom to alter the house however they want. The issue for me is that my walls are plastered and easy to emulsion. I do not want wall paper as I will probably have to strip it and the painting is then harder (more coats required).

I am happy for them to do what they want with the house if they stay for 10 years but the average stay is 2.5 years.

Who is miserable and mean like me? Who gives them carte blanche?