I don't know how others handle it but usually my rough guide is to take the average number of expected adventurers - say 3-8 level 5 adventurers would be an average of six, then figure out how much treasure it takes to get from level 5 to 6, times that by six, and then adjust accordingly to module size or relative difficulty. If it's the kind of module you think warrants a level up at the end, you're done! Typically I'll half or quarter the total treasure, then divvy that up between coins, valuables, and magic items.