[LazyPup]

This is a very good question. It ultimatly depends upon what code is in force in your locality.

Normally the washing machine riser would be connected near the floor to allow enough vertical height for the mandatory 18" minimum rise on the standpipe. (The trap must be a minimum of 6" above the floor to permit servicing.)

The laundry sink could be tied in at the horizontal height from the sink drain to the vertical waste pipe, taking pitch into consideration. (1/4"/ft)

Under the Internatoinal Residential code when combining two drains at a similar height their is no actual requirement as to which must be above the other.

Under the Uniform Plumbing Code when two waste arms are to be connected to a riser at or near a common point, they MUST BE connected at the same level by means of a back to back fitting (sanitary cross). (UPC 905.6)