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 $29.91   12.216%   $33.5638   $33.56   $33.55   $33.60  
  Purchase #2 $76.97   8.548%   $83.5494   $83.55   $83.55   $83.50  
  Purchase #3 $143.85   11.551%   $160.4661   $160.47   $160.45   $160.50  
  Purchase #4 $27.43   8.507%   $29.7635   $29.76   $29.75   $29.80  
  Purchase #5 $179.48   3.804%   $186.3074   $186.31   $186.30   $186.30  
  Purchase #6 $8.64   4.144%   $8.998   $9.00   $9.00   $9.00  
  Purchase #7 $242.87   5.686%   $256.6796   $256.68   $256.70   $256.70  
  7 purchase totals = $709.15       $759.3278   $759.33   $759.30   $759.40