Old post, but I'd say the cleanest approach would be to add a class to the surrounding div and a button class on each button so your CSS rule becomes useful for more use cases:
/* Added to highlight spacing */_x000D_
.is-grouped { _x000D_
display: inline-block;_x000D_
background-color: yellow;_x000D_
}_x000D_
_x000D_
.is-grouped > .button:not(:last-child) {_x000D_
margin-right: 10px;_x000D_
}
_x000D_
Spacing shown in yellow<br><br>_x000D_
_x000D_
<div class='is-grouped'>_x000D_
<button class='button'>Save</button>_x000D_
<button class='button'>Save As...</button>_x000D_
<button class='button'>Delete</button>_x000D_
</div>
_x000D_