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 $231.40   10.190%   $254.9797   $254.98   $255.00   $255.00  
  Purchase #2 $119.34   9.485%   $130.6594   $130.66   $130.65   $130.70  
  Purchase #3 $31.73   3.407%   $32.811   $32.81   $32.80   $32.80  
  Purchase #4 $156.84   13.662%   $178.2675   $178.27   $178.25   $178.30  
  Purchase #5 $107.28   3.636%   $111.1807   $111.18   $111.20   $111.20  
  Purchase #6 $70.25   5.701%   $74.255   $74.26   $74.25   $74.30  
  Purchase #7 $168.80   4.109%   $175.736   $175.74   $175.75   $175.70  
  7 purchase totals = $885.64       $957.8893   $957.90   $957.90   $958.00