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 $26.94   12.463%   $30.2975   $30.30   $30.30   $30.30  
  Purchase #2 $104.77   11.869%   $117.2052   $117.21   $117.20   $117.20  
  Purchase #3 $66.48   6.686%   $70.9249   $70.92   $70.90   $70.90  
  Purchase #4 $60.86   5.836%   $64.4118   $64.41   $64.40   $64.40  
  Purchase #5 $156.88   12.729%   $176.8493   $176.85   $176.85   $176.80  
  Purchase #6 $240.45   5.575%   $253.8551   $253.86   $253.85   $253.90  
  Purchase #7 $41.28   7.304%   $44.2951   $44.30   $44.30   $44.30  
  7 purchase totals = $697.66       $757.8389   $757.85   $757.80   $757.80