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 $250.32   7.747%   $269.7123   $269.71   $269.70   $269.70  
  Purchase #2 $150.29   4.356%   $156.8366   $156.84   $156.85   $156.80  
  Purchase #3 $167.57   5.522%   $176.8232   $176.82   $176.80   $176.80  
  Purchase #4 $15.62   13.164%   $17.6762   $17.68   $17.70   $17.70  
  Purchase #5 $107.73   9.189%   $117.6293   $117.63   $117.65   $117.60  
  Purchase #6 $54.32   5.469%   $57.2908   $57.29   $57.30   $57.30  
  Purchase #7 $8.57   4.738%   $8.976   $8.98   $9.00   $9.00  
  7 purchase totals = $754.42       $804.9444   $804.95   $805.00   $804.90