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 $6.39   7.757%   $6.8857   $6.89   $6.90   $6.90  
  Purchase #2 $75.41   3.757%   $78.2432   $78.24   $78.25   $78.20  
  Purchase #3 $144.89   7.305%   $155.4742   $155.47   $155.45   $155.50  
  Purchase #4 $222.92   5.625%   $235.4593   $235.46   $235.45   $235.50  
  Purchase #5 $11.59   4.378%   $12.0974   $12.10   $12.10   $12.10  
  Purchase #6 $176.94   3.619%   $183.3435   $183.34   $183.35   $183.30  
  Purchase #7 $147.71   13.241%   $167.2683   $167.27   $167.25   $167.30  
  7 purchase totals = $785.85       $838.7716   $838.77   $838.75   $838.80