Whole top sirloin

As long as it is whole muscle without cuts or flaps you need to hold in together, tying would just be for appearances only.  Only tie if you need to hold it's shape and/or form into a precise shape, size or configuration.  For example, a rump roast tied up:


That brings together the top, eye, and bottom round pieces into one unit.

A whole top sirloin untied would look like this:


which is already bound together by its natural seams and would not require tying up.