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 $19.80   9.822%   $21.7448   $21.74   $21.75   $21.70  
  Purchase #2 $108.76   11.221%   $120.964   $120.96   $120.95   $121.00  
  Purchase #3 $163.25   12.856%   $184.2374   $184.24   $184.25   $184.20  
  Purchase #4 $52.91   6.363%   $56.2767   $56.28   $56.30   $56.30  
  Purchase #5 $47.31   12.129%   $53.0482   $53.05   $53.05   $53.00  
  Purchase #6 $151.50   9.448%   $165.8137   $165.81   $165.80   $165.80  
  Purchase #7 $102.73   10.233%   $113.2424   $113.24   $113.25   $113.20  
  7 purchase totals = $646.26       $715.3272   $715.32   $715.35   $715.20