For instance, gold demand is notoriously soft during June and July. If you were looking to buy gold around May, it might make sense to wait until mid-July. Not always, but often. In equities, September is typically a poor performing month, but could be a good month to buy on the dip. So the thought is that perhaps the entire permanent portfolio might exhibit those same tendencies. The problem is I don't have the data to run on it. Anyone want to check it out?
One shortcut on this would be to observe the asset classes as a whole. If two or three are performing well together, it might make sense to keep accumulating cash until the portfolio as a whole begins to suffer--at which point it would be wise to buy lagging assets. Of course this applies to those initially looking to deploy into the portfolio from a traditional portfolio or from all cash--not as much those already in the portfolio. In the latter camp it would be a good exercise to observe your rebalance bands.
Anyway, just a random thought....