don't look at just whether you need a box/bag, look at the cost.
some airlines, especially us-based carriers can charge $100-200
to carry a bike. and they may charge that each way. and then
they may charge that for each leg of the flight. check the airline
website...AND PRINT OFF A COPY TO CARRY WITH YOU TO THE
CHECK-IN COUNTER. often, the ground staff don't know the
rules.

here in asialand, some of the carriers (chinese domestic) will
still carry bikes for free. the big budget carriers (airasia, tiger,
jetstar) will charge a sporting goods fee of $15-30 per leg.
the website allows you to pay the bike fee online when you
purchase the tix.

many asian airlines used to be roll-on/roll-off. turn the bars,
remove the pedals, deflate the tires. but most are now requiring
a box. you can occasionally talk your way on board without a
box, or with the saran-wrapping. any city with an airport will
have a bike shop. for $5-10, they'll sell you a box and probably
pack your bike for you. failing that, find a home appliance store.
refrigerator/washer boxes can be cut to fit.