I think it depends on the airline. Last year I bought an Am ser. tele in hongkong and may kasamang hardcase yun so they had to make me check it in. At the counter I asked the lady if I had to check the guitar in if it were in a softcase and she said you can handcarry it. So naisip ko na sayang, dapat nag dala ako ng soft case so I can hand carry the guitar and check in the hardcase. To my luck though the guitar arrived fine. This was Cathay Pacific.
Last Jan naman nagpabili friend ko ng Epi Les Paul sa HK. From my previous experience of what the lady told me over the counter the last time I went, I decided to purchase a softcase for it so I can hand carry it. We used PAL this time. At the counter, the lady says it too big, bulky, need to check it in. We had a small argument, even refering to my previous experience of what was told to me about hand carrying a guitar in its softcase. Of course to keep things at bay, I had to comply. Signed the waiver, put the fragile stickers around it and cleared my throat. Upon arriving, I immediately checked the guitar at the carousel. Alas, a long crack was found along the headstock. Further inspection revealed that the headstock was hanging on because of the tension of the strings. The guitar fell forward in the softcase. I was furious...I did not want to make a scene though. Brought it to Arie the next day to have it fixed and now its almost invisible and guranteed to be stronger than before. My friend was happy.
Based on these experiences all I can say is expect the worse but prepare for the worse. DO NOT CHECK IN A GUITAR IN A SOFTCASE! If a good hardcase costs as much as 1/2 the price of a guitar I'd say go for it. You'll not only be protecting but it will give you peace of mind not only on the trip but for years to come. If a hardcase might be out of budget or incovenient, I suggest you ask for the manufacturers box when you buy the guitar, place the softcase with the guitar in there and wrap the entire case with bubble wrap like what eders19 mentioned then fill the empty spaces with crushed newspaper, stryrofoam or clothes. This will provide more than enough cushoning for the guitar. The box maybe heavier but at least your guitar is protected.
