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 $5.68   12.309%   $6.3792   $6.38   $6.40   $6.40  
  Purchase #2 $19.60   11.645%   $21.8824   $21.88   $21.90   $21.90  
  Purchase #3 $98.50   9.756%   $108.1097   $108.11   $108.10   $108.10  
  Purchase #4 $137.38   13.739%   $156.2546   $156.25   $156.25   $156.30  
  Purchase #5 $197.81   8.217%   $214.064   $214.06   $214.05   $214.10  
  Purchase #6 $36.37   10.269%   $40.1048   $40.10   $40.10   $40.10  
  Purchase #7 $153.40   6.282%   $163.0366   $163.04   $163.05   $163.00  
  7 purchase totals = $648.74       $709.8313   $709.82   $709.85   $709.90