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 $242.52   13.383%   $274.9765   $274.98   $275.00   $275.00  
  Purchase #2 $30.61   13.154%   $34.6364   $34.64   $34.65   $34.60  
  Purchase #3 $43.39   5.585%   $45.8133   $45.81   $45.80   $45.80  
  Purchase #4 $133.81   8.579%   $145.2896   $145.29   $145.30   $145.30  
  Purchase #5 $116.64   7.509%   $125.3985   $125.40   $125.40   $125.40  
  Purchase #6 $109.75   8.315%   $118.8757   $118.88   $118.90   $118.90  
  Purchase #7 $65.48   5.328%   $68.9688   $68.97   $68.95   $69.00  
  7 purchase totals = $742.20       $813.9588   $813.97   $814.00   $814.00