Do checkboxes need labels?
Label tags act as an anchor for all form elements. It not only works for checkboxes, but you should use it for every form element that has a label.
Should checkbox be before or after label?
The Accessibility sites recommend the labels come after the checkbox, and use a "for" attribute. Toolkits usually provide a widget which forces the checkbox to be on the right of the label, so there is no guideline to write.