Of couse it's Ok. I remember when I first started creating Wordpress themes... I found a website on Google (which I cant find right now for the life of me) that gave an overview of which aspects of say, the default Wordpress theme you should strip out before you start editing it. Have a Google if you're struggling on where to start.
On my first wordpress theme I edited the default theme, took a while to get the hang of the wordpress coding, but after I started to understand, it's pretty easy. I never code my wordpress themes from scratch though, I think it's a waste of time. I do the designing on CSS coding from scratch of course, but then I just take my old wordpress themes and start edit from there, since then you got all the nessesary wordpress code and you only need to code the html and css (Sometimes abit of wordpress coding but not much)
Why bother trying to find a theme that you can change instead of making one from the scratch. You can do it in a few minutes if you use WPDesigner's tutorial "So you want to create WordPress themes huh?". Think about it!
I would have to disagree with that. After learning the Wordpress structure I find it much easier to design my template first. Then, once my design is exactly how I want it I add the Wordpress code around it. This way your design is the framework, and Wordpress is tamed by it. Much better IMO.
I just treat it as a content management system and bend the HTML output to my will. I do have a Wordpress master template that I'm working on. When I'm done with it, I'll probably release it under an MIT license (to Hell with the GPL).