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 $58.53   13.479%   $66.4193   $66.42   $66.40   $66.40  
  Purchase #2 $221.63   9.609%   $242.9264   $242.93   $242.95   $242.90  
  Purchase #3 $177.88   12.116%   $199.4319   $199.43   $199.45   $199.40  
  Purchase #4 $25.36   9.266%   $27.7099   $27.71   $27.70   $27.70  
  Purchase #5 $70.97   10.420%   $78.3651   $78.37   $78.35   $78.40  
  Purchase #6 $160.61   13.153%   $181.735   $181.74   $181.75   $181.70  
  Purchase #7 $193.54   9.690%   $212.294   $212.29   $212.30   $212.30  
  7 purchase totals = $908.52       $1,008.8816   $1,008.89   $1,008.90   $1,008.80