I seem to remember years ago that some kit beers did suggest that you boiled the whole volume of the kit. Presumably just because of some sort of paranoia about contamination. Now my Dad taught me how to make beer kits and his view was don't bother. Hence I've never boiled a kit.
I've not seen it on the instructions for any of the kits I do these days.
My method, boil a kettle full of water, pour the malt from the kit into your fermenter, use the boiled water to fill the pouches / cans from the kit to dissolve whatever malt is left, add this to the fermenter and the rest of the kettle of water if there is any left. Give it all a good stir to dissolve all the malt. Boil another kettle, probably doesn't need to be full.
Now add cold (tap) water until getting near the kit volume, give it a good stir, check the temperature, too low add some more boiled water, too high, continue adding cold to the required volume.