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 $136.72   10.823%   $151.5172   $151.52   $151.50   $151.50  
  Purchase #2 $86.56   7.359%   $92.93   $92.93   $92.95   $92.90  
  Purchase #3 $70.48   9.616%   $77.2574   $77.26   $77.25   $77.30  
  Purchase #4 $55.41   11.808%   $61.9528   $61.95   $61.95   $62.00  
  Purchase #5 $77.85   3.751%   $80.7702   $80.77   $80.75   $80.80  
  Purchase #6 $73.62   3.643%   $76.302   $76.30   $76.30   $76.30  
  Purchase #7 $125.50   10.565%   $138.7591   $138.76   $138.75   $138.80  
  7 purchase totals = $626.14       $679.4887   $679.49   $679.45   $679.60