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 $127.71   4.835%   $133.8848   $133.88   $133.90   $133.90  
  Purchase #2 $248.80   8.581%   $270.1495   $270.15   $270.15   $270.10  
  Purchase #3 $97.81   11.452%   $109.0112   $109.01   $109.00   $109.00  
  Purchase #4 $12.67   10.166%   $13.958   $13.96   $13.95   $14.00  
  Purchase #5 $5.70   5.234%   $5.9983   $6.00   $6.00   $6.00  
  Purchase #6 $75.60   6.768%   $80.7166   $80.72   $80.70   $80.70  
  Purchase #7 $26.61   12.572%   $29.9554   $29.96   $29.95   $30.00  
  7 purchase totals = $594.90       $643.6738   $643.68   $643.65   $643.70