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 $185.98   11.389%   $207.1613   $207.16   $207.15   $207.20  
  Purchase #2 $60.37   5.446%   $63.6578   $63.66   $63.65   $63.70  
  Purchase #3 $124.95   8.252%   $135.2609   $135.26   $135.25   $135.30  
  Purchase #4 $86.56   13.485%   $98.2326   $98.23   $98.25   $98.20  
  Purchase #5 $122.68   5.745%   $129.728   $129.73   $129.75   $129.70  
  Purchase #6 $101.62   12.838%   $114.666   $114.67   $114.65   $114.70  
  Purchase #7 $89.36   4.322%   $93.2221   $93.22   $93.20   $93.20  
  7 purchase totals = $771.52       $841.9287   $841.93   $841.90   $842.00