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 $128.61   5.276%   $135.3955   $135.40   $135.40   $135.40  
  Purchase #2 $240.61   3.314%   $248.5838   $248.58   $248.60   $248.60  
  Purchase #3 $91.40   6.545%   $97.3821   $97.38   $97.40   $97.40  
  Purchase #4 $6.67   5.696%   $7.0499   $7.05   $7.05   $7.00  
  Purchase #5 $173.89   10.672%   $192.4475   $192.45   $192.45   $192.40  
  Purchase #6 $6.97   9.160%   $7.6085   $7.61   $7.60   $7.60  
  Purchase #7 $13.83   12.224%   $15.5206   $15.52   $15.50   $15.50  
  7 purchase totals = $661.98       $703.9879   $703.99   $704.00   $703.90