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 $13.88   10.747%   $15.3717   $15.37   $15.35   $15.40  
  Purchase #2 $166.67   9.502%   $182.507   $182.51   $182.50   $182.50  
  Purchase #3 $42.74   7.671%   $46.0186   $46.02   $46.00   $46.00  
  Purchase #4 $29.32   4.686%   $30.6939   $30.69   $30.70   $30.70  
  Purchase #5 $38.72   7.814%   $41.7456   $41.75   $41.75   $41.70  
  Purchase #6 $61.61   13.821%   $70.1251   $70.13   $70.15   $70.10  
  Purchase #7 $106.65   9.604%   $116.8927   $116.89   $116.90   $116.90  
  7 purchase totals = $459.59       $503.3546   $503.36   $503.35   $503.30