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 $34.96   13.201%   $39.5751   $39.58   $39.60   $39.60  
  Purchase #2 $167.27   12.126%   $187.5532   $187.55   $187.55   $187.60  
  Purchase #3 $62.89   13.656%   $71.4783   $71.48   $71.50   $71.50  
  Purchase #4 $86.25   8.626%   $93.6899   $93.69   $93.70   $93.70  
  Purchase #5 $172.52   3.247%   $178.1217   $178.12   $178.10   $178.10  
  Purchase #6 $185.61   12.386%   $208.5997   $208.60   $208.60   $208.60  
  Purchase #7 $14.38   10.135%   $15.8374   $15.84   $15.85   $15.80  
  7 purchase totals = $723.88       $794.8553   $794.86   $794.90   $794.90