No problem at all. If you have any doubt - even if using all whole leaves - stir it and water it. It will break down quickly if you mix and keep wet. That can be done when applied as mulch.

Whole tree leaves can mat and can block water flow, and can make growing difficult for plants that die back to the soil, if that is all that is put down as mulch. A mix of shredded and whole leaves will help alleviate some of that problem, however.

I agree with Kimmer for the most part. Leaves also blow away should they get dry. I seem to be one of the few who put fencing around leaves which I gather each fall before the garbage trucks arrive. I have been getting 50 to 100 bags I suppose. its taking me three years per pile and there is a lot of shrinkage, but it is a most beautiful soil and a product completely transformed in a biological process which assures a soil of lively vitality. When I can get around to it I'd rather mulch with wood chips from the town or power company.

Whole leaves are easily picked up by winds and carried off while shredded leaves are less likely to be blown away unless the winds are quite high, just as soil particles can be blown about by the wind.

A solution that I have used is to double shredd the leaves. If you are using a mower, dump the collection bag and run over and collect the leaves a second time. That should chop them up enoungh to use them for mulch. If you have a lot of wind, lay a wire mesh (I use a wire fence mesh) over the shredded leaves to keep them from blowing away.