You can do whatever you want if it is accurate enough for the work you are doing.
Does your method really have any advantages though? Squaring a vice only takes a minute or two. Cheap edge-finders have garbage repeatability.
With a DTI you are measuring a line as you move across the vice jaw. If you are edge finding, you are measuring only two points on the vice jaw which has a higher potential for error. Any DTI will match or beat the accuracy of an edge finder.
There is no real reason why you shouldn't use G68 instead of squaring your workpiece, but it seems like it would save very little time and potentially cost a lot of accuracy.