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 $18.88   9.848%   $20.7393   $20.74   $20.75   $20.70  
  Purchase #2 $229.65   13.468%   $260.5793   $260.58   $260.60   $260.60  
  Purchase #3 $99.88   6.146%   $106.0186   $106.02   $106.00   $106.00  
  Purchase #4 $110.39   8.748%   $120.0469   $120.05   $120.05   $120.00  
  Purchase #5 $141.95   6.354%   $150.9695   $150.97   $150.95   $151.00  
  Purchase #6 $86.87   4.272%   $90.5811   $90.58   $90.60   $90.60  
  Purchase #7 $132.74   5.629%   $140.2119   $140.21   $140.20   $140.20  
  7 purchase totals = $820.36       $889.1466   $889.15   $889.15   $889.10