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 $26.97   3.752%   $27.9819   $27.98   $28.00   $28.00  
  Purchase #2 $226.73   6.281%   $240.9709   $240.97   $240.95   $241.00  
  Purchase #3 $172.57   3.693%   $178.943   $178.94   $178.95   $178.90  
  Purchase #4 $77.31   8.191%   $83.6425   $83.64   $83.65   $83.60  
  Purchase #5 $119.66   10.614%   $132.3607   $132.36   $132.35   $132.40  
  Purchase #6 $123.58   10.709%   $136.8142   $136.81   $136.80   $136.80  
  Purchase #7 $30.43   6.767%   $32.4892   $32.49   $32.50   $32.50  
  7 purchase totals = $777.25       $833.2024   $833.19   $833.20   $833.20