Yes wilton instructors teach to do the writing first. The reason for that is that sometimes when you get your cake all decorated you don't always have enough room for the message. I tell my students to invision their completed cake in their head so they can see where they want the message to go. After you have been decorating cakes for a while it will get easier. I joke with my students that if they are decorating a cake when I am watching to do it my way but when they are at home do what is easiest for them. LOL
My Wilton instructor told us also when writing and decorating, just to make sure we have enough room to write what we need to on the cake. I think it's a matter of preference myself.
I do my writting last, probably because it is my least favorite part and that is the way my mom always did it. I should switch it though. My writting is really bad and I usually end up scraping it off a few times. I don't know why but it never occurred to me to do it first. 
BEFOREEEEEE
everytime i forget and i have to do it after i regret it..i have to watch out for flowers, borders, pictures...whatever...if u do it first, u have a blank slate...gives you space to move your bag ans tip to make the writing come out nice and then u decorate around that...i'm not saying write all over the place..u still have to know where everythings going...but when u write on an undecorated cake, i think its easier
Wow, I always do the writing last (my least favorite thing). That's probably why I do so many cakes with the tops heavily decorated that there is no room to write anything! It is probably a good idea to write before and I will start practicing my writing so that it looks half way decent! Best of luck to you.
I do my decorating, then my writing, then the border last. In most of my cakes, the writing isn't as important as the picture. And I almost always PRINT, not write, I have better control matching the size of each letter to the last one.
I always do my writing after - to me the decorations are the centerpiece, and the design is the most important -the words I will fit in wherever they fit later. I suppose on a sheet cake type cake it would be different, and then writing first would make sense. I guess each person finds their own technique!
I've tried doing it both ways and I've decided that it really doesn't matter either way so long as you have enough space to finish your decorating and enough space for the writing.
I prefer to do any top decoration first, then the writing, then the borders and side decs if I'm doing them, I've run out of writing space too many times when leaving it until last!
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