Nope, it's not necessary. I rarely do a mashout. Normally I go straight from mash to boil, also. While I'm sparging, the wort is heating up in the boil kettle. When I'm done sparging, that goes straight into the boil also. Cuts my time way down, and the resultant beer tastes good either way. If I'm trying to do a beer for a competition, then I'll mash out "properly"; otherwise, mash to boil works for me.