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 $20.33   4.435%   $21.2316   $21.23   $21.25   $21.20  
  Purchase #2 $231.28   5.759%   $244.5994   $244.60   $244.60   $244.60  
  Purchase #3 $49.84   11.757%   $55.6997   $55.70   $55.70   $55.70  
  Purchase #4 $16.45   11.500%   $18.3418   $18.34   $18.35   $18.30  
  Purchase #5 $25.91   7.334%   $27.8102   $27.81   $27.80   $27.80  
  Purchase #6 $171.56   6.417%   $182.569   $182.57   $182.55   $182.60  
  Purchase #7 $106.83   6.583%   $113.8626   $113.86   $113.85   $113.90  
  7 purchase totals = $622.20       $664.1143   $664.11   $664.10   $664.10