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 $245.66   13.683%   $279.2737   $279.27   $279.25   $279.30  
  Purchase #2 $244.57   12.175%   $274.3464   $274.35   $274.35   $274.30  
  Purchase #3 $88.66   5.665%   $93.6826   $93.68   $93.70   $93.70  
  Purchase #4 $182.63   13.271%   $206.8668   $206.87   $206.85   $206.90  
  Purchase #5 $186.86   6.696%   $199.3721   $199.37   $199.35   $199.40  
  Purchase #6 $76.81   7.327%   $82.4379   $82.44   $82.45   $82.40  
  Purchase #7 $97.43   9.685%   $106.8661   $106.87   $106.85   $106.90  
  7 purchase totals = $1,122.62       $1,242.8456   $1,242.85   $1,242.80   $1,242.90