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 $11.62   7.380%   $12.4776   $12.48   $12.50   $12.50  
  Purchase #2 $229.58   10.585%   $253.881   $253.88   $253.90   $253.90  
  Purchase #3 $28.32   3.783%   $29.3913   $29.39   $29.40   $29.40  
  Purchase #4 $78.95   6.687%   $84.2294   $84.23   $84.25   $84.20  
  Purchase #5 $68.71   4.539%   $71.8287   $71.83   $71.85   $71.80  
  Purchase #6 $221.84   3.763%   $230.1878   $230.19   $230.20   $230.20  
  Purchase #7 $154.55   12.406%   $173.7235   $173.72   $173.70   $173.70  
  7 purchase totals = $793.57       $855.7193   $855.72   $855.80   $855.70