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 $170.55   4.626%   $178.4396   $178.44   $178.45   $178.40  
  Purchase #2 $17.25   9.627%   $18.9107   $18.91   $18.90   $18.90  
  Purchase #3 $116.89   7.147%   $125.2441   $125.24   $125.25   $125.20  
  Purchase #4 $93.74   12.703%   $105.6478   $105.65   $105.65   $105.60  
  Purchase #5 $55.85   9.245%   $61.0133   $61.01   $61.00   $61.00  
  Purchase #6 $176.67   7.872%   $190.5775   $190.58   $190.60   $190.60  
  Purchase #7 $7.83   11.177%   $8.7052   $8.71   $8.70   $8.70  
  7 purchase totals = $638.78       $688.5382   $688.54   $688.55   $688.40