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 $28.56   9.325%   $31.2232   $31.22   $31.20   $31.20  
  Purchase #2 $28.51   12.857%   $32.1755   $32.18   $32.20   $32.20  
  Purchase #3 $249.76   11.777%   $279.1742   $279.17   $279.15   $279.20  
  Purchase #4 $212.77   4.276%   $221.868   $221.87   $221.85   $221.90  
  Purchase #5 $20.52   8.242%   $22.2113   $22.21   $22.20   $22.20  
  Purchase #6 $146.53   4.310%   $152.8454   $152.85   $152.85   $152.80  
  Purchase #7 $140.33   13.239%   $158.9083   $158.91   $158.90   $158.90  
  7 purchase totals = $826.98       $898.4059   $898.41   $898.35   $898.40