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 $25.88   13.126%   $29.277   $29.28   $29.30   $29.30  
  Purchase #2 $10.47   13.151%   $11.8469   $11.85   $11.85   $11.80  
  Purchase #3 $154.83   7.211%   $165.9948   $165.99   $166.00   $166.00  
  Purchase #4 $236.46   6.517%   $251.8701   $251.87   $251.85   $251.90  
  Purchase #5 $247.99   8.597%   $269.3097   $269.31   $269.30   $269.30  
  Purchase #6 $9.29   6.798%   $9.9215   $9.92   $9.90   $9.90  
  Purchase #7 $61.39   5.604%   $64.8303   $64.83   $64.85   $64.80  
  7 purchase totals = $746.31       $803.0503   $803.05   $803.05   $803.00