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 $5.26   5.148%   $5.5308   $5.53   $5.55   $5.50  
  Purchase #2 $153.77   9.411%   $168.2413   $168.24   $168.25   $168.20  
  Purchase #3 $157.89   11.451%   $175.97   $175.97   $175.95   $176.00  
  Purchase #4 $100.45   5.621%   $106.0963   $106.10   $106.10   $106.10  
  Purchase #5 $71.79   10.255%   $79.1521   $79.15   $79.15   $79.20  
  Purchase #6 $105.86   3.437%   $109.4984   $109.50   $109.50   $109.50  
  Purchase #7 $18.38   4.373%   $19.1838   $19.18   $19.20   $19.20  
  7 purchase totals = $613.40       $663.6727   $663.67   $663.70   $663.70