Having the date and PO# as part of your lot number will help you to make sense of the information later. Having the system generate numbers (I think there is an add-on program that can do that for you) may sound easy to implement, but the numbers can be pretty meaningless six months down the road. You'd have to go back and research the PO and the date in the system to know where that lot came from.  
 
Similarly, by using SO-N to connect Sales Orders to Work Orders, and then using the Work Order number as the Lot number for finished goods, we now know (when a customer calls with a problem) a lot about the history of the product before we start looking it up in Evo. The lot number tells us the order numbers and gives us an idea of when the product was made. Coded lot numbers are peachy keen.