I find that generally speaking, the best use of miles is for upgrades. That way you pay the 'taxes' (mainly 'airline imposed surcharges') for the lower cabin fare, which is what the killer is for award tickets. The upgrade does not attract extra surcharges and usually no extra taxes, the notable exception being travel out of UK airports where the APD for any cabin higher than the cheapest one is significantly more than for the cheapest one (economy) and the airline will collect this difference. There may be other examples of these additional charges but I am not aware of them.If you have paid for a J ticket and upgrade to F, there is no additional APD.
So apart from the above mentioned anomaly, upgrading is 'cash free'. Of course upgrades need to be available and that can (for some routes) be quite a problem. I (and mrs t) have had pretty decent results asking for Y to J upgrades in the lounge at DXB for CPT flights which are notoriously difficult to find the 'normal' way (in advance). But you have to be prepared to be disappointed and sit 'in the back'