I dont think that there is such a thing as "intact" DNA - only minimally sheared DNA. DNA will naturally have breaks in it due to damage, repair process etc - these can be single or double stranded. Most extraction processes will involve DNA being subject to shear forces and when resolved on a gel will give rise to a smear of different sizes. The "least sheared" DNA can be obtained when cells are embedded intact in agarose blocks,, digested in the blocks and then resolved using field inversion gel electrophoresis. However extracting the DNA from the agarose without further damaging it is another matter ...
Immediate Heatdenaturation combined with SDS Inactivation before addition of Proteinase K is a safe procedure. Don´t forget EDTA in the denaturation step