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 $134.92   3.584%   $139.7555   $139.76   $139.75   $139.80  
  Purchase #2 $80.62   12.859%   $90.9869   $90.99   $91.00   $91.00  
  Purchase #3 $207.41   10.166%   $228.4953   $228.50   $228.50   $228.50  
  Purchase #4 $28.27   5.672%   $29.8735   $29.87   $29.85   $29.90  
  Purchase #5 $32.95   5.427%   $34.7382   $34.74   $34.75   $34.70  
  Purchase #6 $71.96   8.553%   $78.1147   $78.11   $78.10   $78.10  
  Purchase #7 $28.94   3.868%   $30.0594   $30.06   $30.05   $30.10  
  7 purchase totals = $585.07       $632.0235   $632.03   $632.00   $632.10