In general it is recommended to do "Album" scans, because in this case both TrackGain and AlbumGain values are calculated. Then you can select whether you want to use Track or Album optimised playback. When you select TrackGain all tracks on the album will have approx. the same volume level, while albumGain sustains the difference between the more loud and more quiet songs on the album.
For classical music replaygain may not be necessery, but it doesn't hurt, because in the case when no volume adjustion is needed the RG value will be set to 0.00 dB