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 $18.62   12.767%   $20.9972   $21.00   $21.00   $21.00  
  Purchase #2 $9.64   9.782%   $10.583   $10.58   $10.60   $10.60  
  Purchase #3 $211.37   3.792%   $219.3852   $219.39   $219.40   $219.40  
  Purchase #4 $70.74   3.611%   $73.2944   $73.29   $73.30   $73.30  
  Purchase #5 $164.55   11.820%   $183.9998   $184.00   $184.00   $184.00  
  Purchase #6 $35.67   12.500%   $40.1288   $40.13   $40.15   $40.10  
  Purchase #7 $121.91   10.212%   $134.3594   $134.36   $134.35   $134.40  
  7 purchase totals = $632.50       $682.7478   $682.75   $682.80   $682.80