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 $28.30   6.461%   $30.1285   $30.13   $30.15   $30.10  
  Purchase #2 $33.85   13.476%   $38.4116   $38.41   $38.40   $38.40  
  Purchase #3 $122.37   3.114%   $126.1806   $126.18   $126.20   $126.20  
  Purchase #4 $105.91   7.845%   $114.2186   $114.22   $114.20   $114.20  
  Purchase #5 $144.95   9.262%   $158.3753   $158.38   $158.40   $158.40  
  Purchase #6 $163.27   9.624%   $178.9831   $178.98   $179.00   $179.00  
  Purchase #7 $103.81   7.706%   $111.8096   $111.81   $111.80   $111.80  
  7 purchase totals = $702.46       $758.1073   $758.11   $758.15   $758.10