I have used 333 thinner as cleaning agent just before applying coat of varnish.  Does thin varnish as you cover the area so I only use in last 2 or 3 coats to not reduce varnish build up.  I usually apply between 7 and 12 coats as base varnish.  The last two coats are sem-annual scraficial coats.  I recoat all exposed areas every 6 to 8 months in Texas heat and humidiy.  (Of course, covered or protected or not continously exposed areas don't get the same treatment.)

I always use 333 thinner before each coat of varnish or urethane. It cleans off any errant fingerprints or dust. It is so volatile, however, that there is no way that enough would remain on the surface long enough to thin the varnish. -Wes

I have used 333 for the past 40+ yrs!  It is a solvent which is primarily used to increase flow in hot weather. 216 is a better solvent in my opinion and 333 for smoothing flow of brightside urethane and varnish. 

Using it to pre-clean surface will not thin varnish. 

dave