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 $20.72   4.564%   $21.6657   $21.67   $21.65   $21.70  
  Purchase #2 $68.36   11.369%   $76.1318   $76.13   $76.15   $76.10  
  Purchase #3 $24.56   5.275%   $25.8555   $25.86   $25.85   $25.90  
  Purchase #4 $37.97   3.529%   $39.31   $39.31   $39.30   $39.30  
  Purchase #5 $132.96   7.337%   $142.7153   $142.72   $142.70   $142.70  
  Purchase #6 $26.37   7.103%   $28.2431   $28.24   $28.25   $28.20  
  Purchase #7 $229.36   8.769%   $249.4726   $249.47   $249.45   $249.50  
  7 purchase totals = $540.30       $583.3940   $583.40   $583.35   $583.40