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 $64.59   13.842%   $73.5305   $73.53   $73.55   $73.50  
  Purchase #2 $14.71   10.683%   $16.2815   $16.28   $16.30   $16.30  
  Purchase #3 $8.72   10.111%   $9.6017   $9.60   $9.60   $9.60  
  Purchase #4 $124.91   6.130%   $132.567   $132.57   $132.55   $132.60  
  Purchase #5 $56.83   8.127%   $61.4486   $61.45   $61.45   $61.40  
  Purchase #6 $187.67   5.830%   $198.6112   $198.61   $198.60   $198.60  
  Purchase #7 $87.37   7.695%   $94.0931   $94.09   $94.10   $94.10  
  7 purchase totals = $544.80       $586.1336   $586.13   $586.15   $586.10