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 $123.94   12.674%   $139.6482   $139.65   $139.65   $139.60  
  Purchase #2 $188.77   13.391%   $214.0482   $214.05   $214.05   $214.00  
  Purchase #3 $41.59   10.183%   $45.8251   $45.83   $45.85   $45.80  
  Purchase #4 $146.93   4.838%   $154.0385   $154.04   $154.05   $154.00  
  Purchase #5 $81.33   5.546%   $85.8406   $85.84   $85.85   $85.80  
  Purchase #6 $103.60   12.754%   $116.8131   $116.81   $116.80   $116.80  
  Purchase #7 $64.27   5.772%   $67.9797   $67.98   $68.00   $68.00  
  7 purchase totals = $750.43       $824.1934   $824.20   $824.25   $824.00