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 $81.81   8.193%   $88.5127   $88.51   $88.50   $88.50  
  Purchase #2 $166.25   11.783%   $185.8392   $185.84   $185.85   $185.80  
  Purchase #3 $183.65   6.546%   $195.6717   $195.67   $195.65   $195.70  
  Purchase #4 $115.41   11.411%   $128.5794   $128.58   $128.60   $128.60  
  Purchase #5 $158.53   11.565%   $176.864   $176.86   $176.85   $176.90  
  Purchase #6 $176.74   4.730%   $185.0998   $185.10   $185.10   $185.10  
  Purchase #7 $54.70   10.854%   $60.6371   $60.64   $60.65   $60.60  
  7 purchase totals = $937.09       $1,021.2039   $1,021.20   $1,021.20   $1,021.20