Pros: it's faster.
Cons: with RAID5 you can have horrible corruption failures with write cache, when kernel thinks everything is written to disk, and it is safe to continue. Basically, you can get your whole array destroyed unrecoverably (or actually, it is possible to recover, but it is huge mess). We had that case earlier when battery backup for write caches failed (raid card memory backup), and it wasn't nice.
Cons: small performance hit for writing.
Pros: your data is safer.
Your usage looks like mainly reading (you aren't adding new CD's/DVD's all the time, are you, same goes for family photos etc.). Is it important to have little bit more performance for writing?
How long it will take to rip all data again, and restore all backups in case of failure? If that is trivial amount of time (or someone else will do it :)) go for it, but if you really do not want to do that, I would skip adding small performance improvement.
Also, you can enable write cache temporarily, and run some benchmarks, for example bonnie++ or just copy'n'paste some files to disk to see if there is any advantage. If you can't notice it, is it worth of risks?