How should I prepare the coffee to be used in my tiramisu?
Someone will be able to notice.  In matters of taste, there is always variation.  Some people can't taste the difference between stale Folgers and fresh high quality coffee.  However, many can.  

I the case of your dessert, without more data, I can't tell if you would be able to tell or not.  But, for sure, someone would be able to tell.  That being the case, it's worth doing with the best ingredients you can reasonably use.  

My personal opinion for my cooking is any ingredient that isn't high enough quality to stand on it's own shouldn't get used.