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 $212.46   12.652%   $239.3404   $239.34   $239.35   $239.30  
  Purchase #2 $44.45   3.501%   $46.0062   $46.01   $46.00   $46.00  
  Purchase #3 $69.47   10.167%   $76.533   $76.53   $76.55   $76.50  
  Purchase #4 $68.43   13.517%   $77.6797   $77.68   $77.70   $77.70  
  Purchase #5 $151.34   9.785%   $166.1486   $166.15   $166.15   $166.10  
  Purchase #6 $250.63   12.251%   $281.3347   $281.33   $281.35   $281.30  
  Purchase #7 $130.92   8.711%   $142.3244   $142.32   $142.30   $142.30  
  7 purchase totals = $927.70       $1,029.3670   $1,029.36   $1,029.40   $1,029.20