This is really dependent on the look that the bride is seeking to achieve. Some styles are as simple as curling the hair and leaving it down and some brides want a very elegant and formal hair style to go with their wedding day theme. When it comes to something like this, then I would say that a hair stylist is necessary to perfect and achieve the look you are going for. If you are on a budget and have a relatively pretty easy hairstyle, I would recommend that you attempt to practice it and see if you are able to do it yourself.
As a hair/makeup artist vendor sometimes I have brides that want very simple styles and although they are simple for us, it may not be for them and so they have us assist them with it. But I would say, if you can do it, go for it! :)
I agree with Susie as I myself am a hair stylist and makeup artist. Most brides want something simple elegant and easy. Of course they cant do it themselves that is why I as a stylist help them get the look they want. I give them ideas. At the trial I do about 2-3 hairstyles on them and they decide on what they like and or want.
if they want to try something themselves then by all means go for it.