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 $135.95   10.247%   $149.8808   $149.88   $149.90   $149.90  
  Purchase #2 $229.47   4.191%   $239.0871   $239.09   $239.10   $239.10  
  Purchase #3 $201.32   6.136%   $213.673   $213.67   $213.65   $213.70  
  Purchase #4 $208.25   12.672%   $234.6394   $234.64   $234.65   $234.60  
  Purchase #5 $114.65   6.657%   $122.2823   $122.28   $122.30   $122.30  
  Purchase #6 $143.81   12.726%   $162.1113   $162.11   $162.10   $162.10  
  Purchase #7 $110.26   6.120%   $117.0079   $117.01   $117.00   $117.00  
  7 purchase totals = $1,143.71       $1,238.6818   $1,238.68   $1,238.70   $1,238.70