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 $233.49   8.598%   $253.5655   $253.57   $253.55   $253.60  
  Purchase #2 $76.26   4.789%   $79.9121   $79.91   $79.90   $79.90  
  Purchase #3 $183.26   4.266%   $191.0779   $191.08   $191.10   $191.10  
  Purchase #4 $245.51   5.467%   $258.932   $258.93   $258.95   $258.90  
  Purchase #5 $38.34   9.605%   $42.0226   $42.02   $42.00   $42.00  
  Purchase #6 $135.88   6.452%   $144.647   $144.65   $144.65   $144.60  
  Purchase #7 $25.89   3.731%   $26.856   $26.86   $26.85   $26.90  
  7 purchase totals = $938.63       $997.0131   $997.02   $997.00   $997.00