If you feel more comfortable using 434 then it won't matter at all while playing this piece. However, the very fact that you have said that you prefer to use 434 suggests there may be a possibility that the reason you prefer this is because these fingers feel stronger than 545. The question is, would you always prefer to avoid weaker fingers, or would it be useful to strengthen up the 5th finger to the point where it wouldn't make the slightest difference to you to use 434 or 545? If you are able to use 545 equally as well as 434, would you even be asking this question? My thoughts on this are that your teacher is right - you should use 545 if only for the reason that you will be strengthening your little finger in preparation for other pieces you might play in the future. My view is, if there is a weakness, don't avoid it - work it out! You have 5 useful fingers to use on each hand - why limit this to only 4?