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 $8.53   5.347%   $8.9861   $8.99   $9.00   $9.00  
  Purchase #2 $158.83   8.196%   $171.8477   $171.85   $171.85   $171.80  
  Purchase #3 $9.97   8.522%   $10.8196   $10.82   $10.80   $10.80  
  Purchase #4 $203.28   13.676%   $231.0806   $231.08   $231.10   $231.10  
  Purchase #5 $122.70   5.482%   $129.4264   $129.43   $129.45   $129.40  
  Purchase #6 $224.60   4.171%   $233.9681   $233.97   $233.95   $234.00  
  Purchase #7 $191.97   6.208%   $203.8875   $203.89   $203.90   $203.90  
  7 purchase totals = $919.88       $990.0160   $990.03   $990.05   $990.00