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 $24.57   11.793%   $27.4675   $27.47   $27.45   $27.50  
  Purchase #2 $229.53   8.633%   $249.3453   $249.35   $249.35   $249.30  
  Purchase #3 $81.76   4.856%   $85.7303   $85.73   $85.75   $85.70  
  Purchase #4 $150.98   8.550%   $163.8888   $163.89   $163.90   $163.90  
  Purchase #5 $245.32   5.819%   $259.5952   $259.60   $259.60   $259.60  
  Purchase #6 $239.89   8.490%   $260.2567   $260.26   $260.25   $260.30  
  Purchase #7 $204.52   7.768%   $220.4071   $220.41   $220.40   $220.40  
  7 purchase totals = $1,176.57       $1,266.6909   $1,266.71   $1,266.70   $1,266.70