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 $7.99   13.622%   $9.0784   $9.08   $9.10   $9.10  
  Purchase #2 $233.91   12.783%   $263.8107   $263.81   $263.80   $263.80  
  Purchase #3 $42.54   12.434%   $47.8294   $47.83   $47.85   $47.80  
  Purchase #4 $159.43   11.320%   $177.4775   $177.48   $177.50   $177.50  
  Purchase #5 $21.96   8.286%   $23.7796   $23.78   $23.80   $23.80  
  Purchase #6 $165.75   5.555%   $174.9574   $174.96   $174.95   $175.00  
  Purchase #7 $215.65   10.427%   $238.1358   $238.14   $238.15   $238.10  
  7 purchase totals = $847.23       $935.0688   $935.08   $935.15   $935.10