FLAC compression levels are (only) a trade of between encoding time and file size. The decoding time is pretty much independent of compression rate. In the following I will refer to the compression levels 0, ..., 8 as FLAC-0, ..., FLAC-8.
In short: I recommend FLAC-4!
The Easy Solutions
If I don't care about encoding time and since space is ...
Personally, I use compression all over the place...including on the Mater Bus. I think one of the mistaken notions that people harbor about compressors is that you only use them to make things louder. Obviously, it's not everyone, but I find myself constantly reminding people that they don't have to apply "make up gain." [I know you don't need to know this, ...
The way i do it is to ride the faders to balance things before going into eq, de-esser and compressor. Then i put a Brickwall limiter on the submaster and master.
It's timeconsuming, but it's worth it.
You could try the Waves Vocal Rider plug if time is more important than quality in this case.
In video games I primarily use compression when creating individual assets, or mixing dialogue. So basically I use compression for two purposes, as an effect and to smooth out the signal.
When using it as an effect i'm usually trying to make something sound louder than it is. Exaggerating the dynamics even getting some pumping. An example that comes to ...