FLAC is a lossless format so, yes, you can convert files back to WAV as and when required - as long as you saved your FLAC file using the same sample rate and bit depth as the original WAV.
It actually uses a similar process to RAR or ZIP, with the biggest advantage of FLAC being that you can play back the files without having to extract them first. Secondly, should your file somehow become corrupted, you can usually recover a good portion of it.
RAR, on the other hand, provides you with an option to build redundancy into your files by including a recovery record. Obviously this will mean a bigger file, but a bit more peace of mind with it. Personally, the only time I have ever had problems with RAR files is when downloading them or receiving them via e-mail and, even then, have usually managed to recover most of the contents by selecting the "Keep broken files" option.
The cost of hard drive space has come down so much that, unless you are seriously financially challenged, you should have at least one backup copy of all your important files on an external that can be stored off-site (i.e. in a different location). Also, more and more companies are offering indefinite free cloud storage (e.g. ADrive - 50GB), so you should definitely consider this as an additional fail-safe.
In the archiving fraternity we refer to LOCKSS - Lots of copies keeps stuff safe!