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 $20.57   3.424%   $21.2743   $21.27   $21.25   $21.30  
  Purchase #2 $12.67   10.719%   $14.0281   $14.03   $14.05   $14.00  
  Purchase #3 $63.81   8.795%   $69.4221   $69.42   $69.40   $69.40  
  Purchase #4 $126.39   8.361%   $136.9575   $136.96   $136.95   $137.00  
  Purchase #5 $34.63   9.110%   $37.7848   $37.78   $37.80   $37.80  
  Purchase #6 $155.26   6.443%   $165.2634   $165.26   $165.25   $165.30  
  Purchase #7 $35.44   6.798%   $37.8492   $37.85   $37.85   $37.80  
  7 purchase totals = $448.77       $482.5794   $482.57   $482.55   $482.60