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 $16.60   7.275%   $17.8077   $17.81   $17.80   $17.80  
  Purchase #2 $242.86   11.143%   $269.9219   $269.92   $269.90   $269.90  
  Purchase #3 $183.40   9.225%   $200.3187   $200.32   $200.30   $200.30  
  Purchase #4 $91.75   3.130%   $94.6218   $94.62   $94.60   $94.60  
  Purchase #5 $16.39   12.586%   $18.4528   $18.45   $18.45   $18.50  
  Purchase #6 $216.56   6.359%   $230.3311   $230.33   $230.35   $230.30  
  Purchase #7 $237.94   4.473%   $248.5831   $248.58   $248.60   $248.60  
  7 purchase totals = $1,005.50       $1,080.0371   $1,080.03   $1,080.00   $1,080.00