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 $84.36   12.547%   $94.9446   $94.94   $94.95   $94.90  
  Purchase #2 $147.44   9.659%   $161.6812   $161.68   $161.70   $161.70  
  Purchase #3 $125.60   13.836%   $142.978   $142.98   $143.00   $143.00  
  Purchase #4 $219.71   3.260%   $226.8725   $226.87   $226.85   $226.90  
  Purchase #5 $171.66   3.651%   $177.9273   $177.93   $177.95   $177.90  
  Purchase #6 $157.80   12.550%   $177.6039   $177.60   $177.60   $177.60  
  Purchase #7 $7.32   9.528%   $8.0174   $8.02   $8.00   $8.00  
  7 purchase totals = $913.89       $990.0249   $990.02   $990.05   $990.00