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 $19.91   9.598%   $21.821   $21.82   $21.80   $21.80  
  Purchase #2 $117.88   9.413%   $128.976   $128.98   $129.00   $129.00  
  Purchase #3 $106.25   6.243%   $112.8832   $112.88   $112.90   $112.90  
  Purchase #4 $50.70   9.436%   $55.4841   $55.48   $55.50   $55.50  
  Purchase #5 $20.91   4.284%   $21.8058   $21.81   $21.80   $21.80  
  Purchase #6 $170.35   3.661%   $176.5865   $176.59   $176.60   $176.60  
  Purchase #7 $51.64   10.540%   $57.0829   $57.08   $57.10   $57.10  
  7 purchase totals = $537.64       $574.6395   $574.64   $574.70   $574.70