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 $54.62   3.755%   $56.671   $56.67   $56.65   $56.70  
  Purchase #2 $112.63   8.492%   $122.1945   $122.19   $122.20   $122.20  
  Purchase #3 $148.35   12.610%   $167.0569   $167.06   $167.05   $167.10  
  Purchase #4 $99.26   5.435%   $104.6548   $104.65   $104.65   $104.70  
  Purchase #5 $67.93   4.510%   $70.9936   $70.99   $71.00   $71.00  
  Purchase #6 $54.87   10.164%   $60.447   $60.45   $60.45   $60.40  
  Purchase #7 $13.50   5.612%   $14.2576   $14.26   $14.25   $14.30  
  7 purchase totals = $551.16       $596.2754   $596.27   $596.25   $596.40