USA Mains black to device black & Mains red to device white
Mains guy here.  

Nope. To be precise, Ms. Nope.

We often see people take a bog-standard NEMA 5-15 socket and intentionally miswire it so the (tall) neutral pin is actually connected to the opposing hot pole.  That seems to work, when you plug in a computer via the common cord  (NEMA 5-15P to IEC C13).  But like many things that seem to work, it will kill you.   

Anyway, it's rather silly to do, since there's a right way to do that very thing. 

 

You use the 3rd or 4th gal, officially called Ms. Nope or Ms. Winky-Nope.  (they are female, after all).  The NEMA 6 connector is precisely for that, and as you can see, it's the same form-factor (except for the pins, of course; those are keyed.) And that's it; done.  Get a NEMA 6-15P to ICE C13 power cord, and you're good to go. 

Hardwiring, yeah, that's OK

If you make the electrical connection hardwired, inside a junction box, then yes, you can attach appliance white/black to the two 240V hot wires (black/black, black/red, etc.) 

You do not need to use 3-wire supply cable (black-white-red) if you only want 240V.  You can use 2-wire cable (black-white) but you need to re-mark the white wires near each splice point to designate them as hot wires, typically with black tape.  If your wiring is in conduit with individual wires, you can't use white for a hot; use the correct wire color. 

Your wire color codes are green, yellow-green or bare = safety earthing (which is called "ground" in mains; not to be confused with electronics GND/Vss).   In North America, gray and white = neutral, and all else = hot.

Don't eliminate your minimum required 120V outlets

Most houses are built with only the bare minimum number of 120V circuits and outlets - 1 circuit to a bedroom with outlets every 12', 2 20A circuits to the kitchen countertop receps.  This means, if you re-wire one of those circuits to 240V, you will now fall short of the minimum number of 120V outlets.  Can't do that.  So unless your house was over-wired by a genius electrician, it probably means you'll need to run new circuits in the walls for the 240V. 

Multi-wire branch circuits are a 120V alternative

If you are running /3 (black white red) cable anyway, this can be wired as a multi-wire branch circuit.  It is capable of supporting 2 circuit's worth of power on one cable, as well as  both 120V and 240V loads. However, this makes GFCI and AFCI protection more complicated, which are now required on most circuits that support 120V.