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 $249.52   7.511%   $268.2614   $268.26   $268.25   $268.30  
  Purchase #2 $26.40   12.826%   $29.7861   $29.79   $29.80   $29.80  
  Purchase #3 $154.81   13.350%   $175.4771   $175.48   $175.50   $175.50  
  Purchase #4 $39.76   10.477%   $43.9257   $43.93   $43.95   $43.90  
  Purchase #5 $159.35   12.186%   $178.7684   $178.77   $178.75   $178.80  
  Purchase #6 $96.27   4.201%   $100.3143   $100.31   $100.30   $100.30  
  Purchase #7 $24.32   10.129%   $26.7834   $26.78   $26.80   $26.80  
  7 purchase totals = $750.43       $823.3164   $823.32   $823.35   $823.40