Shared Music and Photos
These two libraries will grow larger each year until your 256GB SSD will start to feel a big tight. I would recommend the following strategy:

Music


If you don't mind spending a little bit of money, you could pay for iTunes Match. We have found this an excellent way to share 120gb of music between machines which do not have enough space to store it all. You can keep the original library on the iMac (it has the room) and sync 'via the cloud' using iTunes Match to your rMBP. It caches the files you play, so any of the music that you're not that bothered about won't take up space on your SSD. This also allows you to get access to your music anywhere you have WiFi - not just at home. As you have iPhones and an iPad (or Apple TV if you get one) they can all share the same library with this mechanism at no additional cost and no synching. If you're in the US, this also gets you ad-free iTunes Radio.
If you'd rather not pay, and never play music outside of the house, you can use the iTunes Home Sharing feature as suggested by NEO.
If you'd rather not pay, but you want music outside of the house, then I suggest breaking up your libraries into music that you like, music that your wife likes and music you both like. You can keep the physical files split across the machines by having it mostly on a shared folder from your wife's iMac, but you must take great care to disable the 'Copy files to iTunes Media Folder when adding to library' option in iTunes preferences, otherwise you'll end up with duplicates. This option means you can only play the music that you both like when you are at home as it needs your home WiFi and the iMac to be on. Also, be sure to Sync your iPhones and iPad regularly.


Photos

(I'm assuming you use iPhoto)


Break up your photo libraries into year-by-year. It's unlikely you want to have multiple years of photos on your rMBP, so this will make editing easier too.
Store the by-year iPhoto libraries on the iMac in a shared folder.
Keep the latest library on your rMBP as you're editing it.
Use iCloud Photo Stream to keep your wife's iMac up-to-date with any new photos before you edit/take/red-eye/reject them.


If your iPhoto library is already a 500GB monster (like ours), you'll need to investigate tools to split it up or invest in some external hard drives to back up before you start splicing it.