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 $250.33   11.854%   $280.0041   $280.00   $280.00   $280.00  
  Purchase #2 $159.68   5.164%   $167.9259   $167.93   $167.95   $167.90  
  Purchase #3 $126.28   6.366%   $134.319   $134.32   $134.30   $134.30  
  Purchase #4 $204.66   6.147%   $217.2405   $217.24   $217.25   $217.20  
  Purchase #5 $93.61   12.218%   $105.0473   $105.05   $105.05   $105.00  
  Purchase #6 $220.66   4.317%   $230.1859   $230.19   $230.20   $230.20  
  Purchase #7 $7.45   13.119%   $8.4274   $8.43   $8.45   $8.40  
  7 purchase totals = $1,062.67       $1,143.1501   $1,143.16   $1,143.20   $1,143.00