I do it without worries, remembering ProfDan's caveat about chemicals.
A typical residential septic tank is usually about 4.5 feet wide x 8.0 feet long and has about a 3 inch drop from inlet to outlet. That means that, if you plugged the outlet, your tank has "headroom" for about 67 gallons of additional effluent.
That said, unlike a cesspool, a septic tank is not a static system. The outlet pipe is as big as the inlet pipe so, unless the leach field is plugged, the tank is usually close to equilibrium. What goes out matches what goes in. Don't forget, you can dump the 45 gallons from your washing machine, the 9-14 gallons from your 30-year old dishwasher, and the 30-ish gallons your teenage daughter ran in the bathtub into your septic tank at the same time.
I know nothing about modern mound systems, though, and can't comment on their effluent throughput capacity.
Florida Keys and far Northern Vermont
2009 Fun Finder XT245
2013 GMC Sierra