I should think the answer is obvious: when the Nobles is dead in hand and it's to your advantage to get a Province that way, and most obviously when it's for the win. Here's
me 2 days ago:
---------- Gherald: turn 19 ----------
Gherald - plays Nobles
Gherald - draws Plaza, Nobles, Silver
Gherald - plays 2 Silver, 2 Copper
Gherald - buys Farmland
Gherald - trashes Nobles
Gherald - gains Province
Gherald - gains Farmland
Gherald - draws Plaza, Copper, Crossroads, Estate, Curse
------------ Game Over ------------
Gherald - cards: 5 Nobles, 3 Plaza, 2 Crossroads, 1 Tournament, 1 Trusty Steed, 7 Copper, 2 Silver, 5 Province, 3 Estate, 2 Farmland, 2 Duchy, 2 Curse
Gherald - total victory points: 51
2.71828..... - cards: 5 Tournament, 3 Native Village, 3 Crossroads, 3 Plaza, 2 Nobles, 1 Followers, 1 Outpost, 1 Bag of Gold, 1 Princess, 7 Copper, 1 Gold, 1 Silver, 7 Estate, 3 Province
2.71828..... - total victory points: 29