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 $246.59   7.580%   $265.2815   $265.28   $265.30   $265.30  
  Purchase #2 $158.94   6.168%   $168.7434   $168.74   $168.75   $168.70  
  Purchase #3 $172.55   12.585%   $194.2654   $194.27   $194.25   $194.30  
  Purchase #4 $126.86   5.626%   $133.9971   $134.00   $134.00   $134.00  
  Purchase #5 $88.75   11.667%   $99.1045   $99.10   $99.10   $99.10  
  Purchase #6 $16.28   12.635%   $18.337   $18.34   $18.35   $18.30  
  Purchase #7 $106.45   10.151%   $117.2557   $117.26   $117.25   $117.30  
  7 purchase totals = $916.42       $996.9846   $996.99   $997.00   $997.00