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 $9.45   4.390%   $9.8649   $9.86   $9.85   $9.90  
  Purchase #2 $8.73   3.420%   $9.0286   $9.03   $9.05   $9.00  
  Purchase #3 $154.72   7.260%   $165.9527   $165.95   $165.95   $166.00  
  Purchase #4 $5.28   13.242%   $5.9792   $5.98   $6.00   $6.00  
  Purchase #5 $8.97   9.282%   $9.8026   $9.80   $9.80   $9.80  
  Purchase #6 $193.48   9.658%   $212.1663   $212.17   $212.15   $212.20  
  Purchase #7 $209.68   4.395%   $218.8954   $218.90   $218.90   $218.90  
  7 purchase totals = $590.31       $631.6897   $631.69   $631.70   $631.80