Its entirely your choice. I don't bother with a top sheet as I can't see the point in having an extra thing to wash and change.As long as you change your duvet cover regularly I don't think you need one.
I use a top sheet.Being a lady of a certain age I find the man made fibres in the duvet make me perspire. Or sweat if you like !! So I always have a cotton sheet under the duvet. I can put a fresh sheet on then every couple of days if I am hot and bothered.Easier for me than changing a duvet cover every couple of days.
After spending time in U S and having such a comfortable and wonderful sleep I have abandoned duvets altogether and now have cotton blankets over a top sheet.
Personally I can't be doing with a top sheet with a duvet. My Mum always had top sheets and I always seemed to end up with the sheet twisted round my legs somehow. Have just changed to a duck down duvet and it is sooooooo much better than a polyester one as it seems to mould it's self to you rather than just lying on top of the bed, definately wouldn't have a top sheet with this.