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 $246.96   13.486%   $280.265   $280.27   $280.25   $280.30  
  Purchase #2 $75.34   3.248%   $77.787   $77.79   $77.80   $77.80  
  Purchase #3 $183.51   5.281%   $193.2012   $193.20   $193.20   $193.20  
  Purchase #4 $159.25   5.537%   $168.0677   $168.07   $168.05   $168.10  
  Purchase #5 $84.63   7.263%   $90.7767   $90.78   $90.80   $90.80  
  Purchase #6 $94.91   13.234%   $107.4704   $107.47   $107.45   $107.50  
  Purchase #7 $144.79   11.128%   $160.9022   $160.90   $160.90   $160.90  
  7 purchase totals = $989.39       $1,078.4702   $1,078.48   $1,078.45   $1,078.60