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 $58.91   11.773%   $65.8455   $65.85   $65.85   $65.80  
  Purchase #2 $135.33   5.472%   $142.7353   $142.74   $142.75   $142.70  
  Purchase #3 $250.80   11.865%   $280.5574   $280.56   $280.55   $280.60  
  Purchase #4 $82.78   12.573%   $93.1879   $93.19   $93.20   $93.20  
  Purchase #5 $122.33   11.721%   $136.6683   $136.67   $136.65   $136.70  
  Purchase #6 $91.45   8.427%   $99.1565   $99.16   $99.15   $99.20  
  Purchase #7 $94.77   5.669%   $100.1425   $100.14   $100.15   $100.10  
  7 purchase totals = $836.37       $918.2934   $918.31   $918.30   $918.30