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 $240.31   12.350%   $269.9883   $269.99   $270.00   $270.00  
  Purchase #2 $57.65   8.117%   $62.3295   $62.33   $62.35   $62.30  
  Purchase #3 $167.58   3.584%   $173.5861   $173.59   $173.60   $173.60  
  Purchase #4 $178.67   6.202%   $189.7511   $189.75   $189.75   $189.80  
  Purchase #5 $185.49   10.416%   $204.8106   $204.81   $204.80   $204.80  
  Purchase #6 $48.88   9.181%   $53.3677   $53.37   $53.35   $53.40  
  Purchase #7 $236.65   7.407%   $254.1787   $254.18   $254.20   $254.20  
  7 purchase totals = $1,115.23       $1,208.0120   $1,208.02   $1,208.05   $1,208.10