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 $19.55   7.205%   $20.9586   $20.96   $20.95   $21.00  
  Purchase #2 $29.80   8.776%   $32.4152   $32.42   $32.40   $32.40  
  Purchase #3 $230.78   8.786%   $251.0563   $251.06   $251.05   $251.10  
  Purchase #4 $19.49   9.110%   $21.2655   $21.27   $21.25   $21.30  
  Purchase #5 $242.38   13.687%   $275.5546   $275.55   $275.55   $275.60  
  Purchase #6 $109.58   13.575%   $124.4555   $124.46   $124.45   $124.50  
  Purchase #7 $120.89   12.664%   $136.1995   $136.20   $136.20   $136.20  
  7 purchase totals = $772.47       $861.9052   $861.92   $861.85   $862.10