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 $103.91   4.435%   $108.5184   $108.52   $108.50   $108.50  
  Purchase #2 $102.48   10.384%   $113.1215   $113.12   $113.10   $113.10  
  Purchase #3 $95.35   11.469%   $106.2857   $106.29   $106.30   $106.30  
  Purchase #4 $11.74   4.125%   $12.2243   $12.22   $12.20   $12.20  
  Purchase #5 $94.62   5.463%   $99.7891   $99.79   $99.80   $99.80  
  Purchase #6 $52.34   6.758%   $55.8771   $55.88   $55.90   $55.90  
  Purchase #7 $19.79   7.508%   $21.2758   $21.28   $21.30   $21.30  
  7 purchase totals = $480.23       $517.0919   $517.10   $517.10   $517.10