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 $30.50   3.320%   $31.5126   $31.51   $31.50   $31.50  
  Purchase #2 $250.94   6.168%   $266.418   $266.42   $266.40   $266.40  
  Purchase #3 $72.25   9.524%   $79.1311   $79.13   $79.15   $79.10  
  Purchase #4 $71.79   11.157%   $79.7996   $79.80   $79.80   $79.80  
  Purchase #5 $174.55   13.272%   $197.7163   $197.72   $197.70   $197.70  
  Purchase #6 $7.73   11.347%   $8.6071   $8.61   $8.60   $8.60  
  Purchase #7 $122.93   5.309%   $129.4564   $129.46   $129.45   $129.50  
  7 purchase totals = $730.69       $792.6411   $792.65   $792.60   $792.60