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 $10.70   13.664%   $12.162   $12.16   $12.15   $12.20  
  Purchase #2 $73.61   8.450%   $79.83   $79.83   $79.85   $79.80  
  Purchase #3 $171.99   3.588%   $178.161   $178.16   $178.15   $178.20  
  Purchase #4 $10.73   11.684%   $11.9837   $11.98   $12.00   $12.00  
  Purchase #5 $25.39   7.351%   $27.2564   $27.26   $27.25   $27.30  
  Purchase #6 $136.26   3.721%   $141.3302   $141.33   $141.35   $141.30  
  Purchase #7 $170.55   11.419%   $190.0251   $190.03   $190.05   $190.00  
  7 purchase totals = $599.23       $640.7484   $640.75   $640.80   $640.80