Cash Reserves

I am curious how much cash reserve I should have in my business. I have read that 3 to 6 months worth of monthly expenses is a good rule of thumb but I am curious if this includes variable COGS expenses. I assume the number one reason for cash reserves is to prepare for a major slump in sales. But when sales slow down so too would material costs and since they're on net 30 and since I am quite aggressive with my receivables (average is 5 days), should these variable costs really be included in the reserves?