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 $57.96   12.282%   $65.0786   $65.08   $65.10   $65.10  
  Purchase #2 $127.72   12.729%   $143.9775   $143.98   $144.00   $144.00  
  Purchase #3 $164.98   7.687%   $177.662   $177.66   $177.65   $177.70  
  Purchase #4 $194.77   7.177%   $208.7486   $208.75   $208.75   $208.70  
  Purchase #5 $8.90   7.119%   $9.5336   $9.53   $9.55   $9.50  
  Purchase #6 $16.77   4.586%   $17.5391   $17.54   $17.55   $17.50  
  Purchase #7 $239.85   10.451%   $264.9167   $264.92   $264.90   $264.90  
  7 purchase totals = $810.95       $887.4561   $887.46   $887.50   $887.40