Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $28.68   8.632%   $31.1557   $31.16   $31.15   $31.20  
  Purchase #2 $68.44   13.484%   $77.6684   $77.67   $77.65   $77.70  
  Purchase #3 $118.51   12.752%   $133.6224   $133.62   $133.60   $133.60  
  Purchase #4 $233.50   10.519%   $258.0619   $258.06   $258.05   $258.10  
  Purchase #5 $97.36   8.103%   $105.2491   $105.25   $105.25   $105.20  
  Purchase #6 $154.91   3.413%   $160.1971   $160.20   $160.20   $160.20  
  Purchase #7 $85.79   5.557%   $90.5574   $90.56   $90.55   $90.60  
  7 purchase totals = $787.19       $856.5120   $856.52   $856.45   $856.60