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 $233.86   6.547%   $249.1708   $249.17   $249.15   $249.20  
  Purchase #2 $152.75   10.300%   $168.4833   $168.48   $168.50   $168.50  
  Purchase #3 $71.99   10.812%   $79.7736   $79.77   $79.75   $79.80  
  Purchase #4 $86.87   12.471%   $97.7036   $97.70   $97.70   $97.70  
  Purchase #5 $94.40   13.432%   $107.0798   $107.08   $107.10   $107.10  
  Purchase #6 $149.77   7.325%   $160.7407   $160.74   $160.75   $160.70  
  Purchase #7 $35.31   7.440%   $37.9371   $37.94   $37.95   $37.90  
  7 purchase totals = $824.95       $900.8889   $900.88   $900.90   $900.90