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 $17.58   7.436%   $18.8872   $18.89   $18.90   $18.90  
  Purchase #2 $97.87   9.644%   $107.3086   $107.31   $107.30   $107.30  
  Purchase #3 $13.97   3.261%   $14.4256   $14.43   $14.45   $14.40  
  Purchase #4 $192.79   13.311%   $218.4523   $218.45   $218.45   $218.50  
  Purchase #5 $69.90   10.524%   $77.2563   $77.26   $77.25   $77.30  
  Purchase #6 $124.95   11.601%   $139.4454   $139.45   $139.45   $139.40  
  Purchase #7 $88.48   11.276%   $98.457   $98.46   $98.45   $98.50  
  7 purchase totals = $605.54       $674.2324   $674.25   $674.25   $674.30