That is true and a great way to preserve berries to use in jam making later.  I do it all the time.  Too hot to make jam in the summer and freeze all my berries and make it when needed.  
If you know you are going to use the berries for jam and know what kind of pectin or recipe you are using it works well to mash and measure the berries like you would if you were making the jam for fresh berries, then freeze the mashed berries.  That way you know you have the right amount of berries and makes it easy when you are ready to use them.  It also takes up less space in the freezer if mashed first.  
If you freeze them whole they will be easy to mash when thawed.  
Good luck.  The jam tastes just like fresh from frozen berries

Thanks for using Ask Extension. 

Nellie Oehler

Dear Nellie,