If prices are rising, I would choose to use the FIFO (first-in, first-out) cost flow assumption. This is because under FIFO, the earliest inventory items purchased are assumed to be sold first, leaving the more recently purchased, and therefore higher-priced, items in inventory. As a result, the cost of goods sold (COGS) will be based on the lower, earlier purchase prices, resulting in higher net income and, therefore, a higher year-end bonus.
Hope this helps :)