Bootstrap 3 Gutter Size

You can keep the default behaviour (with gutter) and add a class to your CSS stylesheet for tasks like yours:

.no-gutter > [class*='col-'] {
    padding-right:0;
    padding-left:0;
}

And here’s how you can use it in your HTML:

<div class="row no-gutter">
    <div class="col-md-4">
        ...
    </div>
    <div class="col-md-4">
        ...
    </div>
    <div class="col-md-4">
        ...
    </div>
</div>

I don't think Bass's answer is correct. Why touch the row margins? They have a negative margin to offset the column padding for the columns on the edge of the row. Messing with this will break any nested rows.

The answer is simple, just make the container padding equal to the gutter size:

e.g for default bootstrap:

.container {
    padding-left:30px;
    padding-right:30px;
}

http://jsfiddle.net/3wBE3/61/

try:

.row {
    margin-left: 0;
    margin-right: 0;
}

Every column have a padding of 15 px on both sides. Which makes a gutter between of 30 px. In the case of the sm-grid your container class will 970px (((940px + @grid-gutter-width))). Every column get a width of 940/12. The resulting @grid-gutter-width/2 on both sides of the grid will be hide with a negative margin of - 15px;. Undoing this negative left-margin set a gutter of 30 px on both sides of the grid. This gutter is build with 15px padding of the column + 15 px resting grid space.

update

In response of the answer of @ElwoodP, consider the follow code:

<div class="container" style="background-color:darkblue;">  
<div class="row" style="background-color:yellow;">
  <div class="col-md-9" style="background-color:green;">
    <div style="background-color:lightblue;">div 1: .col-md-9</div>
    <div class="row" style="background-color:orange;">
      <div class="col-md-6" style="background-color:red;">
        <div style="background-color:lightblue;">div 2: .col-md-6</div>
      </div>
      <div class="col-md-6" style="background-color:red;">
        <div style="background-color:lightblue;">div 2: .col-md-6</div>
      </div>
    </div>
  </div>  
  <div class="col-md-3" style="background-color:green;">
    <div style="background-color:lightblue;">div 1: .col-md-3</div>
  </div>      
</div>
</div>

In the case of nesting, manipulation the .row class indeed influences the sub grid. Good or fault depends on your expectations / requirements for the subgrid. Changing the margin of the .row won't break the sub grid.

default:

margin of the .row class

with:

.row {
    margin-left: 0;
    margin-right: 0;
}

padding of the .container class

with:

.container {
    padding-left:30px;
    padding-right:30px;
}

Notice sub grids shouldn't wrapped inside a .container class.