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 $239.27   12.364%   $268.8533   $268.85   $268.85   $268.90  
  Purchase #2 $82.69   3.340%   $85.4518   $85.45   $85.45   $85.50  
  Purchase #3 $103.63   12.247%   $116.3216   $116.32   $116.30   $116.30  
  Purchase #4 $81.98   3.122%   $84.5394   $84.54   $84.55   $84.50  
  Purchase #5 $185.37   10.472%   $204.7819   $204.78   $204.80   $204.80  
  Purchase #6 $30.27   9.634%   $33.1862   $33.19   $33.20   $33.20  
  Purchase #7 $53.78   11.841%   $60.1481   $60.15   $60.15   $60.10  
  7 purchase totals = $776.99       $853.2823   $853.28   $853.30   $853.30