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 $107.49   7.412%   $115.4572   $115.46   $115.45   $115.50  
  Purchase #2 $91.81   10.797%   $101.7227   $101.72   $101.70   $101.70  
  Purchase #3 $139.54   3.818%   $144.8676   $144.87   $144.85   $144.90  
  Purchase #4 $12.31   7.604%   $13.2461   $13.25   $13.25   $13.20  
  Purchase #5 $153.53   9.838%   $168.6343   $168.63   $168.65   $168.60  
  Purchase #6 $7.96   13.285%   $9.0175   $9.02   $9.00   $9.00  
  Purchase #7 $82.94   4.472%   $86.6491   $86.65   $86.65   $86.60  
  7 purchase totals = $595.58       $639.5945   $639.60   $639.55   $639.50