Yr wyf bob amser wedi haeru y dylai cyllideb Llywodraeth Cymru ysgwyddo'r risgiau ac elwa ar effaith ei phenderfyniadau ar drethi datganoledig. Serch hynny, os yw polisi trethi yng Nghymru’n parhau’r un fath ag yng ngweddill y DU ac os bydd twf  mewn refeniw y pen o drethi datganoledig yn cyfateb i’r twf yng ngweddill y DU, ni ddylai fod unrhyw effaith ar gyllideb Llywodraeth Cymru. Mae hynny'n gyson â chasgliadau Comisiwn Silk a'r egwyddor "no detriment" a amlinellwyd gan Gomisiwn Smith ar gyfer yr Alban.
I have always maintained that the Welsh Government’s budget should bear the risks and reap the rewards from the impact of its decisions on devolved taxes.  Nevertheless, if tax policy in Wales remains the same as in the rest of the UK and growth in revenues per head from devolved taxes matches that in the rest of the UK, the Welsh Government’s budget should be unaffected.  That is consistent with the conclusions from the Silk Commission and the "no detriment" principle outlined by the Smith Commission for Scotland.