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 $248.91   13.162%   $281.6715   $281.67   $281.65   $281.70  
  Purchase #2 $74.27   3.627%   $76.9638   $76.96   $76.95   $77.00  
  Purchase #3 $117.66   11.257%   $130.905   $130.91   $130.90   $130.90  
  Purchase #4 $170.95   12.157%   $191.7324   $191.73   $191.75   $191.70  
  Purchase #5 $209.61   11.757%   $234.2538   $234.25   $234.25   $234.30  
  Purchase #6 $167.88   6.155%   $178.213   $178.21   $178.20   $178.20  
  Purchase #7 $225.47   8.491%   $244.6147   $244.61   $244.60   $244.60  
  7 purchase totals = $1,214.75       $1,338.3542   $1,338.34   $1,338.30   $1,338.40