When to convert to log in a Multiple Regression

So i have it in my notes that when calculating a multiple regression the independent variables should be converted to their log and then converted back at the end. I was doing some EOC questions and the book calculated the regression using the numbers given for the independent variables. They did not convert the independent variables to their log… why is this? Your help is greatly appreciated.

It depends on whether or not the trend displays exponential growth.

They will also let you know in the question whether they have used logs or not.