Every change you make has an impact. Cause and effect. Do you know what affect your changes are making? As we discuss elsewhere at Performancing, even the seemingly trivial changes can have massive impacts on usability, traffic and most off all advertising click throughs. What might look like a work of art to you and your template designer might be a massive turnoff to your audience.
I’m not trying to put you off meddling with your site! My advice is whatever changes you make, monitor the changes in your metrics and keep the good changes and rollback the bad. Most of all, determine WHY something works or not. (You do look at your stats, don’t you?)
Perhaps this AListApart article can explain better than I can. In the article Nick Usborne shows three designs for the same page, with startlingly different results.