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 $248.46   11.295%   $276.5236   $276.52   $276.50   $276.50  
  Purchase #2 $171.58   13.113%   $194.0793   $194.08   $194.10   $194.10  
  Purchase #3 $24.58   13.680%   $27.9425   $27.94   $27.95   $27.90  
  Purchase #4 $45.84   7.405%   $49.2345   $49.23   $49.25   $49.20  
  Purchase #5 $182.26   5.654%   $192.565   $192.57   $192.55   $192.60  
  Purchase #6 $94.54   4.129%   $98.4436   $98.44   $98.45   $98.40  
  Purchase #7 $146.93   7.505%   $157.9571   $157.96   $157.95   $158.00  
  7 purchase totals = $914.19       $996.7456   $996.74   $996.75   $996.70