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 $26.49   3.629%   $27.4513   $27.45   $27.45   $27.50  
  Purchase #2 $35.73   7.691%   $38.478   $38.48   $38.50   $38.50  
  Purchase #3 $53.67   9.243%   $58.6307   $58.63   $58.65   $58.60  
  Purchase #4 $97.32   12.247%   $109.2388   $109.24   $109.25   $109.20  
  Purchase #5 $33.83   12.326%   $37.9999   $38.00   $38.00   $38.00  
  Purchase #6 $66.32   5.137%   $69.7269   $69.73   $69.75   $69.70  
  Purchase #7 $139.96   13.794%   $159.2661   $159.27   $159.25   $159.30  
  7 purchase totals = $453.32       $500.7917   $500.80   $500.85   $500.80