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 $34.83   13.594%   $39.5648   $39.56   $39.55   $39.60  
  Purchase #2 $247.25   7.247%   $265.1682   $265.17   $265.15   $265.20  
  Purchase #3 $108.93   4.610%   $113.9517   $113.95   $113.95   $114.00  
  Purchase #4 $173.96   5.665%   $183.8148   $183.81   $183.80   $183.80  
  Purchase #5 $157.52   12.279%   $176.8619   $176.86   $176.85   $176.90  
  Purchase #6 $94.73   3.637%   $98.1753   $98.18   $98.20   $98.20  
  Purchase #7 $233.40   5.290%   $245.7469   $245.75   $245.75   $245.70  
  7 purchase totals = $1,050.62       $1,123.2836   $1,123.28   $1,123.25   $1,123.40