Now I can answer better knowing from what basis you ask! Any saving in using reverse idle is counterbalanced by increased brake wear. I believe most airlines find the most cost effective method is to use reverse idle only- that way reverse is available in seconds if really needed. I believe the Qantas 747 accident at BKK was made worse by not habitually selecting even reverse idle. Few shorthaul airlines use more than idle reverse- noise is a big factor too. Fuel counters only click down in 10s of kgs. It is also available only from nose down to 6o kts anyway when reverse idle must be selected, so not for long. So I don't believe a pilot can answer as the only difference is a clicking down faster for a few seconds, but nobody is looking at fuel gauges at that stage. Reverse power is not 'efficient' reverse- it makes lots of noise for relatively little effect.