me as a DM... I don't. with less then a handful of exceptions (including the above ring that ended a campaign, and a holy avenger I out right told the rogue he couldn't sell instead of letting the paladin have it) always let the players decid. that is the issue though. the players normally take "who can get the most out of it" more then "how can we balance the game" even if they complain about the balance.
in the boots of flying example the idea of a wizard getting flight and still having a concentration spell up will almost ALWAYS take it over a fighter getting to fly... even if for months the fighter has complained about too many flying advaceries...
WORST of all is if an argument breaks out "I need flight to keep up" "I can be invisable in the air dropping fireballs and that is WAY better then you closeing with some wyvern knights" and then people take sides and we loose a night of RP to arguments that bleed in and out of game as people take sides.