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 $226.49   3.530%   $234.4851   $234.49   $234.50   $234.50  
  Purchase #2 $113.70   9.301%   $124.2752   $124.28   $124.30   $124.30  
  Purchase #3 $57.30   12.349%   $64.376   $64.38   $64.40   $64.40  
  Purchase #4 $161.70   7.211%   $173.3602   $173.36   $173.35   $173.40  
  Purchase #5 $54.83   3.861%   $56.947   $56.95   $56.95   $56.90  
  Purchase #6 $115.55   8.631%   $125.5231   $125.52   $125.50   $125.50  
  Purchase #7 $99.85   4.481%   $104.3243   $104.32   $104.30   $104.30  
  7 purchase totals = $829.42       $883.2909   $883.30   $883.30   $883.30