Common classes for margin and padding

I have declared common css classes for common margin and padding classes in my css so that i can use them without making other css declarations too specific.

For example :

.padTB5{padding:5px 0;} 
.pad10{padding:10px;}
.mTop10{margin:10px 0 0;} 
.mTop5{margin:5px 0 0;}

1. Is this method right??
2. If not then why?
3. If yes then which is better margin or padding? (I know margin issues with browsers)

Please guide... thanks in advance :)