Like all things, if you have open space the tendency is to fill it. I moved to a MacBook Air with 64gig drive and I wondered if I would really be able to use the machine. For my work, I only really needed 30 gigs of apps/cad/documents. I only accessed my media files at home or on my Iphone. I also became very efficient about deleting files no longer needed. Worked fine for me for 2 years.
I recently purchase a Retina Macbook Pro with 256gig ssd. This is a huge jump up. Over the last month, I have actually gone down on my usage to 28 gigs. Unless my work requirements change, I will have enough room for quite some time. I find I actually like keeping all my media files on my home network where they are safe. My music collection is on my IPhone. And if I am using my MacBook Pro, I usually prefer to listen to Pandora or check out YouTube for music.
I do not synch or back up my Iphone with Retina MacBook Pro. I use my MacMini for that to save a little space.
Just look at what you need, while you are away from home base on a daily basis. If it fits comfortably in a 256, then you are safe. If you are using more than half, I would be a little worried and probably get the larger size initially if you can afford it.
If not as others have stated you can always upgrade.
Good Luck