Your pond will start to silt up, literally with the very first fill up.
If you have lots of dirty surface runoff (plowed fields, etc.) or lots of leaf material (or other organics) blowing in, then your pond will silt up at a significantly faster rate than "ideal" conditions.
Still having at least 8' of max depth - five years in the future, will make the fish in your mature pond much happier.
No one wants to spend money on pointless items. However, I think most of the experts on Pond Boss would tell you that some extra depth is probably a very good investment.
My amateur advice, would be to keep adding some depth as long as the earthmoving is relatively easy going for you or the contractor. If you hit bedrock, or dig through your bottom clay into a sand layer, then back off - cover it with some lifts of good clay, and compact it in.
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