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 $142.33   11.719%   $159.0097   $159.01   $159.00   $159.00  
  Purchase #2 $167.61   8.186%   $181.3306   $181.33   $181.35   $181.30  
  Purchase #3 $156.44   9.285%   $170.9655   $170.97   $170.95   $171.00  
  Purchase #4 $33.71   8.123%   $36.4483   $36.45   $36.45   $36.40  
  Purchase #5 $113.62   10.665%   $125.7376   $125.74   $125.75   $125.70  
  Purchase #6 $193.36   5.250%   $203.5114   $203.51   $203.50   $203.50  
  Purchase #7 $32.33   5.138%   $33.9911   $33.99   $34.00   $34.00  
  7 purchase totals = $839.40       $910.9942   $911.00   $911.00   $910.90