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 $248.96   10.694%   $275.5838   $275.58   $275.60   $275.60  
  Purchase #2 $205.98   10.372%   $227.3442   $227.34   $227.35   $227.30  
  Purchase #3 $16.25   10.792%   $18.0037   $18.00   $18.00   $18.00  
  Purchase #4 $115.66   12.624%   $130.2609   $130.26   $130.25   $130.30  
  Purchase #5 $137.26   4.691%   $143.6989   $143.70   $143.70   $143.70  
  Purchase #6 $60.30   4.192%   $62.8278   $62.83   $62.85   $62.80  
  Purchase #7 $48.26   12.679%   $54.3789   $54.38   $54.40   $54.40  
  7 purchase totals = $832.67       $912.0982   $912.09   $912.15   $912.10