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 $246.57   5.551%   $260.2571   $260.26   $260.25   $260.30  
  Purchase #2 $97.34   5.674%   $102.8631   $102.86   $102.85   $102.90  
  Purchase #3 $191.96   10.609%   $212.325   $212.33   $212.35   $212.30  
  Purchase #4 $247.32   13.216%   $280.0058   $280.01   $280.00   $280.00  
  Purchase #5 $84.46   13.561%   $95.9136   $95.91   $95.90   $95.90  
  Purchase #6 $145.63   3.622%   $150.9047   $150.90   $150.90   $150.90  
  Purchase #7 $163.84   8.759%   $178.1907   $178.19   $178.20   $178.20  
  7 purchase totals = $1,177.12       $1,280.4600   $1,280.46   $1,280.45   $1,280.50