Correct way to create rounded corners in Twitter Bootstrap

<div class="img-rounded"> will give you rounded corners.

I guess it is what you are looking for: http://blogsh.de/tag/bootstrap-less/

@import 'bootstrap.less';
div.my-class {
    .border-radius( 5px );
}

You can use it because there is a mixin:

.border-radius(@radius: 5px) {
  -webkit-border-radius: @radius;
     -moz-border-radius: @radius;
          border-radius: @radius;
}

For Bootstrap 3, there are 4 mixins you can use...

.border-top-radius(@radius);
.border-right-radius(@radius);
.border-bottom-radius(@radius);
.border-left-radius(@radius);

or you can make your own mixin using the top 4 to do it in one shot.

.border-radius(@radius){
    .border-top-radius(@radius);
    .border-right-radius(@radius);
    .border-bottom-radius(@radius);
    .border-left-radius(@radius);
}

Bootstrap is just a big, useful, yet simple CSS file - not a framework or anything you can't override. I say this because I've noticed many developers got stick with BS classes and became lazy "I-can't-write-CSS-code-anymore" coders [this not being your case of course!].

If it features something you need, go with Bootstrap classes - if not, go write your additional code in good ol' style.css.

To have best of both worlds, you may write your own declarations in LESS and recompile the whole thing upon your needs, minimizing server request as a bonus.