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 $28.70   12.335%   $32.2401   $32.24   $32.25   $32.20  
  Purchase #2 $60.41   10.845%   $66.9615   $66.96   $66.95   $67.00  
  Purchase #3 $147.30   9.775%   $161.6986   $161.70   $161.70   $161.70  
  Purchase #4 $241.51   5.281%   $254.2641   $254.26   $254.25   $254.30  
  Purchase #5 $248.96   12.444%   $279.9406   $279.94   $279.95   $279.90  
  Purchase #6 $89.64   8.494%   $97.254   $97.25   $97.25   $97.30  
  Purchase #7 $116.79   6.537%   $124.4246   $124.42   $124.40   $124.40  
  7 purchase totals = $933.31       $1,016.7835   $1,016.77   $1,016.75   $1,016.80