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 $28.57   4.749%   $29.9268   $29.93   $29.95   $29.90  
  Purchase #2 $25.71   10.771%   $28.4792   $28.48   $28.50   $28.50  
  Purchase #3 $126.49   6.249%   $134.3944   $134.39   $134.40   $134.40  
  Purchase #4 $175.26   13.590%   $199.0778   $199.08   $199.10   $199.10  
  Purchase #5 $28.41   13.757%   $32.3184   $32.32   $32.30   $32.30  
  Purchase #6 $203.87   10.293%   $224.8543   $224.85   $224.85   $224.90  
  Purchase #7 $213.82   7.784%   $230.4637   $230.46   $230.45   $230.50  
  7 purchase totals = $802.13       $879.5146   $879.51   $879.55   $879.60