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 $18.80   12.787%   $21.204   $21.20   $21.20   $21.20  
  Purchase #2 $135.73   7.528%   $145.9478   $145.95   $145.95   $145.90  
  Purchase #3 $79.83   9.796%   $87.6501   $87.65   $87.65   $87.70  
  Purchase #4 $64.81   7.232%   $69.4971   $69.50   $69.50   $69.50  
  Purchase #5 $194.58   12.308%   $218.5289   $218.53   $218.55   $218.50  
  Purchase #6 $53.99   7.163%   $57.8573   $57.86   $57.85   $57.90  
  Purchase #7 $29.70   6.721%   $31.6961   $31.70   $31.70   $31.70  
  7 purchase totals = $577.44       $632.3813   $632.39   $632.40   $632.40