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 $28.92   8.542%   $31.3903   $31.39   $31.40   $31.40  
  Purchase #2 $138.68   7.815%   $149.5178   $149.52   $149.50   $149.50  
  Purchase #3 $156.51   4.288%   $163.2211   $163.22   $163.20   $163.20  
  Purchase #4 $133.38   6.817%   $142.4725   $142.47   $142.45   $142.50  
  Purchase #5 $60.85   11.568%   $67.8891   $67.89   $67.90   $67.90  
  Purchase #6 $180.70   4.867%   $189.4947   $189.49   $189.50   $189.50  
  Purchase #7 $24.77   12.537%   $27.8754   $27.88   $27.90   $27.90  
  7 purchase totals = $723.81       $771.8609   $771.86   $771.85   $771.90