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 $25.30   10.308%   $27.9079   $27.91   $27.90   $27.90  
  Purchase #2 $125.89   13.518%   $142.9078   $142.91   $142.90   $142.90  
  Purchase #3 $14.74   10.850%   $16.3393   $16.34   $16.35   $16.30  
  Purchase #4 $197.32   6.232%   $209.617   $209.62   $209.60   $209.60  
  Purchase #5 $37.58   4.327%   $39.2061   $39.21   $39.20   $39.20  
  Purchase #6 $48.52   10.137%   $53.4385   $53.44   $53.45   $53.40  
  Purchase #7 $66.63   4.859%   $69.8676   $69.87   $69.85   $69.90  
  7 purchase totals = $515.98       $559.2842   $559.30   $559.25   $559.20