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 $16.90   11.652%   $18.8692   $18.87   $18.85   $18.90  
  Purchase #2 $169.78   8.463%   $184.1485   $184.15   $184.15   $184.10  
  Purchase #3 $19.27   10.284%   $21.2517   $21.25   $21.25   $21.30  
  Purchase #4 $40.83   12.751%   $46.0362   $46.04   $46.05   $46.00  
  Purchase #5 $137.27   5.337%   $144.5961   $144.60   $144.60   $144.60  
  Purchase #6 $13.95   12.396%   $15.6792   $15.68   $15.70   $15.70  
  Purchase #7 $178.42   13.774%   $202.9956   $203.00   $203.00   $203.00  
  7 purchase totals = $576.42       $633.5765   $633.59   $633.60   $633.60