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 $249.96   13.597%   $283.9471   $283.95   $283.95   $283.90  
  Purchase #2 $159.60   12.118%   $178.9403   $178.94   $178.95   $178.90  
  Purchase #3 $176.63   3.223%   $182.3228   $182.32   $182.30   $182.30  
  Purchase #4 $107.35   6.523%   $114.3524   $114.35   $114.35   $114.40  
  Purchase #5 $233.40   8.682%   $253.6638   $253.66   $253.65   $253.70  
  Purchase #6 $153.87   8.521%   $166.9813   $166.98   $167.00   $167.00  
  Purchase #7 $173.27   11.136%   $192.5653   $192.57   $192.55   $192.60  
  7 purchase totals = $1,254.08       $1,372.7730   $1,372.77   $1,372.75   $1,372.80