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 $249.61   10.772%   $276.498   $276.50   $276.50   $276.50  
  Purchase #2 $60.93   8.399%   $66.0475   $66.05   $66.05   $66.00  
  Purchase #3 $135.56   4.290%   $141.3755   $141.38   $141.40   $141.40  
  Purchase #4 $191.86   4.437%   $200.3728   $200.37   $200.35   $200.40  
  Purchase #5 $231.52   11.295%   $257.6702   $257.67   $257.65   $257.70  
  Purchase #6 $228.97   10.281%   $252.5104   $252.51   $252.50   $252.50  
  Purchase #7 $140.70   13.128%   $159.1711   $159.17   $159.15   $159.20  
  7 purchase totals = $1,239.15       $1,353.6455   $1,353.65   $1,353.60   $1,353.70