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 $150.99   6.325%   $160.5401   $160.54   $160.55   $160.50  
  Purchase #2 $231.49   7.546%   $248.9582   $248.96   $248.95   $249.00  
  Purchase #3 $156.81   7.165%   $168.0454   $168.05   $168.05   $168.00  
  Purchase #4 $119.55   11.618%   $133.4393   $133.44   $133.45   $133.40  
  Purchase #5 $168.95   7.723%   $181.998   $182.00   $182.00   $182.00  
  Purchase #6 $5.95   7.558%   $6.3997   $6.40   $6.40   $6.40  
  Purchase #7 $154.90   7.720%   $166.8583   $166.86   $166.85   $166.90  
  7 purchase totals = $988.64       $1,066.2390   $1,066.25   $1,066.25   $1,066.20