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 $113.25   13.738%   $128.8083   $128.81   $128.80   $128.80  
  Purchase #2 $188.48   5.220%   $198.3187   $198.32   $198.30   $198.30  
  Purchase #3 $178.48   11.160%   $198.3984   $198.40   $198.40   $198.40  
  Purchase #4 $13.34   13.526%   $15.1444   $15.14   $15.15   $15.10  
  Purchase #5 $176.79   11.591%   $197.2817   $197.28   $197.30   $197.30  
  Purchase #6 $31.70   5.568%   $33.4651   $33.47   $33.45   $33.50  
  Purchase #7 $128.40   13.756%   $146.0627   $146.06   $146.05   $146.10  
  7 purchase totals = $830.44       $917.4793   $917.48   $917.45   $917.50