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 $19.98   6.286%   $21.2359   $21.24   $21.25   $21.20  
  Purchase #2 $21.91   12.582%   $24.6667   $24.67   $24.65   $24.70  
  Purchase #3 $150.32   10.830%   $166.5997   $166.60   $166.60   $166.60  
  Purchase #4 $158.32   12.256%   $177.7237   $177.72   $177.70   $177.70  
  Purchase #5 $96.97   10.435%   $107.0888   $107.09   $107.10   $107.10  
  Purchase #6 $76.51   8.415%   $82.9483   $82.95   $82.95   $82.90  
  Purchase #7 $24.41   12.106%   $27.3651   $27.37   $27.35   $27.40  
  7 purchase totals = $548.42       $607.6282   $607.64   $607.60   $607.60