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 $6.61   5.719%   $6.988   $6.99   $7.00   $7.00  
  Purchase #2 $162.28   12.490%   $182.5488   $182.55   $182.55   $182.50  
  Purchase #3 $121.51   6.543%   $129.4604   $129.46   $129.45   $129.50  
  Purchase #4 $100.35   7.351%   $107.7267   $107.73   $107.75   $107.70  
  Purchase #5 $98.51   7.805%   $106.1987   $106.20   $106.20   $106.20  
  Purchase #6 $64.83   9.350%   $70.8916   $70.89   $70.90   $70.90  
  Purchase #7 $33.27   3.582%   $34.4617   $34.46   $34.45   $34.50  
  7 purchase totals = $587.36       $638.2759   $638.28   $638.30   $638.30