Should I use a wood stain before varnishing my indoor floor?
Stain is just color. While it does provide some degree of sealing, whether you use it or not under varnish is entirely discretionary. 

The primary difference between the two options is that proper stain tends to penetrate the grain more, creating more contrast. Colored varnishes overlay a semi-opaque color that tends to mute grain contrast, leaving a more uniform appearance. Again, this is a matter of prerogative.