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 $236.57   13.267%   $267.9557   $267.96   $267.95   $268.00  
  Purchase #2 $8.33   7.285%   $8.9368   $8.94   $8.95   $8.90  
  Purchase #3 $33.63   7.617%   $36.1916   $36.19   $36.20   $36.20  
  Purchase #4 $112.86   8.107%   $122.0096   $122.01   $122.00   $122.00  
  Purchase #5 $230.83   3.617%   $239.1791   $239.18   $239.20   $239.20  
  Purchase #6 $41.28   10.865%   $45.7651   $45.77   $45.75   $45.80  
  Purchase #7 $105.48   8.352%   $114.2897   $114.29   $114.30   $114.30  
  7 purchase totals = $768.98       $834.3276   $834.34   $834.35   $834.40