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 $248.67   8.836%   $270.6425   $270.64   $270.65   $270.60  
  Purchase #2 $93.87   9.503%   $102.7905   $102.79   $102.80   $102.80  
  Purchase #3 $167.63   4.698%   $175.5053   $175.51   $175.50   $175.50  
  Purchase #4 $45.38   5.519%   $47.8845   $47.88   $47.90   $47.90  
  Purchase #5 $98.76   9.847%   $108.4849   $108.48   $108.50   $108.50  
  Purchase #6 $197.88   3.653%   $205.1086   $205.11   $205.10   $205.10  
  Purchase #7 $69.47   6.708%   $74.13   $74.13   $74.15   $74.10  
  7 purchase totals = $921.66       $984.5463   $984.54   $984.60   $984.50