Fryguy et al - there's lots of way people "do it" and by "do it" I mean make a sale of a bike that's relatively close to fitting you the buyer. Often times they can ask "how tall are you?" "What's your inseam?" - and based on those two very general items can ~70% of the time put you on a bike that's close to perfect and then make it perfect with changes to: seat height, seat angle, seat rail slide as well as different stem (shorter, longer, different angle) and tweaking the stack height. This can work. You can stumble into a perfect fit this way.
The problems that can sabotage this process are plenty: maybe the sales person only asks your height and even though you are 6'3" your inseam is 28 and you're all torso. Maybe the sales guy sells Trek and behind those mowers over there some others Treks, and no matter what your body type is Trek doesn't vary enough in top tube length to work for you. Maybe the sales person simply doesn't care or doesn't realize that a triathlon should be positioned on the bike differently from someone doing a fundraising ride. Maybe if you get a 58 in that bike you leave with what ever is "spec'd" on that bike and they don't do none of that funny swappin' of parts kinda thing. And on, and on, and on.
Even if you're gonna skimp and by a lower end bike in some manufacturers line, and even if that's going to be dressed out in Ultegra or, dare I say it, 105 - then
it's still worth it to get the fit done before hand. This will let you discover comfort and power that you might not even know or feel if you wind up on an ill fitting bike.
Now - there can be issues with the fit too. Do some research on the fitter you plan on going to (one research idea might be to check on F.I.S.T certified fitters right here on ST). Be clear with the fitter BEFORE you go that he/she understands the type of riding & racing you'll be doing, and that you'll leave there not with just a bunch of numbers but some options of bikes that would be right for you.
Hope that helps, Ian
Ian Murray
http://www.TriathlonTrainingSeries.com I like the pursuit of mastery
Twitter - @TriCoachIan