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 $83.91   7.765%   $90.4256   $90.43   $90.45   $90.40  
  Purchase #2 $7.26   3.651%   $7.5251   $7.53   $7.55   $7.50  
  Purchase #3 $141.34   8.831%   $153.8217   $153.82   $153.80   $153.80  
  Purchase #4 $127.82   4.706%   $133.8352   $133.84   $133.85   $133.80  
  Purchase #5 $52.98   7.615%   $57.0144   $57.01   $57.00   $57.00  
  Purchase #6 $35.66   8.381%   $38.6487   $38.65   $38.65   $38.60  
  Purchase #7 $44.45   8.401%   $48.1842   $48.18   $48.20   $48.20  
  7 purchase totals = $493.42       $529.4549   $529.46   $529.50   $529.30