It really depends on how pressed for counter-space you are in your kitchen.
I wouldn't put my iPad anywhere there was danger of it plunging into a sink full of water. And I wouldn't want large drops of blazing cooking oil sprayed over it. But the odd dusting of flour, or splash of marinara sauce is going to do it no harm. I typically put my iPad on a stand in a corner (preferably under a cabinet) and enlarge the text so I can read recipes easily from a distance.
I understand the idea behind the bags - but I don't think from a cost or convenience standpoint they are really worth it. IMHO the biggest hazards to an iPad are being dropped in a sink full of dishwater, or dropped onto an unforgiving hardwood or tile floor, neither of which will be ameliorated by a plastic bag. Take appropriate steps to avoid those last two - and worry less about the occasional (if tasty) smudge on the screen.