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 $129.92   7.311%   $139.4185   $139.42   $139.40   $139.40  
  Purchase #2 $80.63   11.248%   $89.6993   $89.70   $89.70   $89.70  
  Purchase #3 $23.64   6.278%   $25.1241   $25.12   $25.10   $25.10  
  Purchase #4 $186.77   3.461%   $193.2341   $193.23   $193.25   $193.20  
  Purchase #5 $16.88   5.271%   $17.7697   $17.77   $17.75   $17.80  
  Purchase #6 $107.61   9.414%   $117.7404   $117.74   $117.75   $117.70  
  Purchase #7 $34.74   12.507%   $39.0849   $39.08   $39.10   $39.10  
  7 purchase totals = $580.19       $622.0710   $622.06   $622.05   $622.00