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 $242.95   11.452%   $270.7726   $270.77   $270.75   $270.80  
  Purchase #2 $97.81   3.311%   $101.0485   $101.05   $101.05   $101.00  
  Purchase #3 $82.94   3.236%   $85.6239   $85.62   $85.60   $85.60  
  Purchase #4 $92.66   6.470%   $98.6551   $98.66   $98.65   $98.70  
  Purchase #5 $168.69   11.308%   $187.7655   $187.77   $187.75   $187.80  
  Purchase #6 $8.32   13.718%   $9.4613   $9.46   $9.45   $9.50  
  Purchase #7 $49.31   9.591%   $54.0393   $54.04   $54.05   $54.00  
  7 purchase totals = $742.68       $807.3662   $807.37   $807.30   $807.40