The way I look at it (which could be wrong) is as follows:
A. You hate to SELL the I-Fund on a day with a -FV. Why? Because TSP subtracts the FV and gives you less money. And since you sold, you will not be there to recoup the FV correction on the following day. Even if Barclays overestimates the FV, and even if they give it all back the following day, you lose it all because you sold and you're out.
B. The reverse tends to work when you BUY the I-Fund on the day following a +FV. You already know that Barclays has applied the +FV and therefore you are guaranteed to receive the full FV correction on the day that you buy. And since Barclays tends to overestimate the FV, you will likely get more of the FV correction than you deserve. It's a sure thing, a guaranteed rebate, regardless of what the OSM does overnight.
Case in point. Monday the USM rallied, causing a +FV of 28 cents. If you bought the I-Fund on Tuesday, you automatically received the -28 cent rebate. And since the OSM only went up 15 cents on Tuesday, the net result was that you bought in at a -13 cents (-0.54%) rebate. Since Barclays once again overestimated the FV, you got a great deal when you bought in and automatically received the inflated FV correction!