I believe that a wedding trial is extremely important and a good thing to do even if you are completely sure about the look that you want to achieve for your wedding day.
A hair and makeup trial is usually the first opportunity you will have to meet your makeup artist, meaning that you have a familiar face there on the wedding day itself and not someone you have only read about online.
It is a good idea to see what the look is like on yourself as opposed to the models wearing the makeup or hair from the images that you have gathered on Google/Pinterest. That way you can adapt bits to really suit you.
And I usually like to book the trial for about one to two months ahead of the wedding day itself. This way, you know what your hair length will roughly be on the wedding day, and how to best care for your skin ahead of the wedding.
I look forward to meeting you all for your trials this year x