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 $121.67   10.415%   $134.3419   $134.34   $134.35   $134.30  
  Purchase #2 $138.64   12.185%   $155.5333   $155.53   $155.55   $155.50  
  Purchase #3 $138.36   13.868%   $157.5478   $157.55   $157.55   $157.50  
  Purchase #4 $21.37   5.827%   $22.6152   $22.62   $22.60   $22.60  
  Purchase #5 $212.51   12.381%   $238.8209   $238.82   $238.80   $238.80  
  Purchase #6 $21.50   6.844%   $22.9715   $22.97   $22.95   $23.00  
  Purchase #7 $237.49   8.312%   $257.2302   $257.23   $257.25   $257.20  
  7 purchase totals = $891.54       $989.0608   $989.06   $989.05   $988.90