Markup validation, its part of developing websites, but there comes a point to when you put off validation for experience. It’s a very common problem, which side to take, but I always try and look of alternative ways of solving issues, while still validating properly. The main question here though, what standard do you validate against currently, and when does this standard change? There are many standards to write against, but most common are xHTML/ 4.01. Each standard comes with different changes, and clarifies new ‘no-no’s.
I think it’s important to stay consistent, as well as, move forward when a standard comes more prevalent in browsers. I do like the idea of Modernizer, taking on some of this for us, but ultimately, we are responsible for making our code work well in the highest percentage of browsers possible.
So what do you do? How do you base your move to a new validation? Do you prefer in-development environment validation, or live, via an external site? What validation rules are you writing your html against?