the only problem with using bonus exp in helm, wrists etc... is that it is additive not cumulative, so (for example) using a 100% leoric crown with max ruby gem actually only nets you an additional 1.4% exp not 66%
this has already been theorycrafted as some guy thought using all the known tricks of leorics gear, custerian, boon of hoarder etc... worked out he should be receiving 8000% exp because he worked it multiplative, when in fact using everything only added 3.7% extra exp due to the way the additive nature of the exp equation works