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 $16.39   10.247%   $18.0695   $18.07   $18.05   $18.10  
  Purchase #2 $167.71   7.805%   $180.7998   $180.80   $180.80   $180.80  
  Purchase #3 $25.56   4.335%   $26.668   $26.67   $26.65   $26.70  
  Purchase #4 $103.27   7.638%   $111.1578   $111.16   $111.15   $111.20  
  Purchase #5 $27.54   8.775%   $29.9566   $29.96   $29.95   $30.00  
  Purchase #6 $149.95   12.582%   $168.8167   $168.82   $168.80   $168.80  
  Purchase #7 $13.50   9.335%   $14.7602   $14.76   $14.75   $14.80  
  7 purchase totals = $503.92       $550.2286   $550.24   $550.15   $550.40