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 $5.99   9.207%   $6.5415   $6.54   $6.55   $6.50  
  Purchase #2 $72.71   7.746%   $78.3421   $78.34   $78.35   $78.30  
  Purchase #3 $73.30   11.767%   $81.9252   $81.93   $81.95   $81.90  
  Purchase #4 $9.80   13.496%   $11.1226   $11.12   $11.10   $11.10  
  Purchase #5 $91.99   9.728%   $100.9388   $100.94   $100.95   $100.90  
  Purchase #6 $21.25   4.258%   $22.1548   $22.15   $22.15   $22.20  
  Purchase #7 $177.44   6.675%   $189.2841   $189.28   $189.30   $189.30  
  7 purchase totals = $452.48       $490.3091   $490.30   $490.35   $490.20