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 $119.45   4.421%   $124.7309   $124.73   $124.75   $124.70  
  Purchase #2 $71.99   6.257%   $76.4944   $76.49   $76.50   $76.50  
  Purchase #3 $165.63   11.200%   $184.1806   $184.18   $184.20   $184.20  
  Purchase #4 $26.68   9.182%   $29.1298   $29.13   $29.15   $29.10  
  Purchase #5 $123.74   6.598%   $131.9044   $131.90   $131.90   $131.90  
  Purchase #6 $21.52   4.109%   $22.4043   $22.40   $22.40   $22.40  
  Purchase #7 $21.86   13.789%   $24.8743   $24.87   $24.85   $24.90  
  7 purchase totals = $550.87       $593.7187   $593.70   $593.75   $593.70