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 $16.75   9.800%   $18.3915   $18.39   $18.40   $18.40  
  Purchase #2 $126.45   13.766%   $143.8571   $143.86   $143.85   $143.90  
  Purchase #3 $106.69   13.772%   $121.3833   $121.38   $121.40   $121.40  
  Purchase #4 $109.52   9.809%   $120.2628   $120.26   $120.25   $120.30  
  Purchase #5 $157.98   3.635%   $163.7226   $163.72   $163.70   $163.70  
  Purchase #6 $39.25   9.815%   $43.1024   $43.10   $43.10   $43.10  
  Purchase #7 $7.41   3.305%   $7.6549   $7.65   $7.65   $7.70  
  7 purchase totals = $564.05       $618.3746   $618.36   $618.35   $618.50