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 $240.75   12.573%   $271.0195   $271.02   $271.00   $271.00  
  Purchase #2 $7.87   10.339%   $8.6837   $8.68   $8.70   $8.70  
  Purchase #3 $179.81   4.459%   $187.8277   $187.83   $187.85   $187.80  
  Purchase #4 $161.96   4.383%   $169.0587   $169.06   $169.05   $169.10  
  Purchase #5 $102.78   6.262%   $109.2161   $109.22   $109.20   $109.20  
  Purchase #6 $228.67   7.685%   $246.2433   $246.24   $246.25   $246.20  
  Purchase #7 $214.88   12.141%   $240.9686   $240.97   $240.95   $241.00  
  7 purchase totals = $1,136.72       $1,233.0176   $1,233.02   $1,233.00   $1,233.00