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 $12.33   5.211%   $12.9725   $12.97   $12.95   $13.00  
  Purchase #2 $37.72   7.187%   $40.4309   $40.43   $40.45   $40.40  
  Purchase #3 $200.27   10.794%   $221.8871   $221.89   $221.90   $221.90  
  Purchase #4 $48.66   6.242%   $51.6974   $51.70   $51.70   $51.70  
  Purchase #5 $189.56   12.729%   $213.6891   $213.69   $213.70   $213.70  
  Purchase #6 $89.28   13.482%   $101.3167   $101.32   $101.30   $101.30  
  Purchase #7 $121.57   11.143%   $135.1165   $135.12   $135.10   $135.10  
  7 purchase totals = $699.39       $777.1102   $777.12   $777.10   $777.10