IANAA, but would suggest this depends on the purpose of the Financial Forecasting.

If you're trying to run a cashflow projection, you probably need to record cogs in the month the cost is incurred.

If you're trying to determine margins, it may be better to records cogs when they're actually sold - just be sure to factor in any loan costs that may be associated (and ignore 'opportunity costs' from purchasing early, because you may as well pull figures out of thin air).

If the forecasting impacts on Tax, then that probably throws another spanner into the works, which I can't really help with.

Edit - you may also want to post the question over at Chuwe.com - hits their audience nicely.