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 $29.59   6.820%   $31.608   $31.61   $31.60   $31.60  
  Purchase #2 $27.56   8.687%   $29.9541   $29.95   $29.95   $30.00  
  Purchase #3 $224.28   13.389%   $254.3088   $254.31   $254.30   $254.30  
  Purchase #4 $9.30   7.600%   $10.0068   $10.01   $10.00   $10.00  
  Purchase #5 $237.77   9.372%   $260.0538   $260.05   $260.05   $260.10  
  Purchase #6 $182.66   10.109%   $201.1251   $201.13   $201.15   $201.10  
  Purchase #7 $29.99   8.275%   $32.4717   $32.47   $32.45   $32.50  
  7 purchase totals = $741.15       $819.5283   $819.53   $819.50   $819.60