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 $28.89   6.561%   $30.7855   $30.79   $30.80   $30.80  
  Purchase #2 $175.95   12.289%   $197.5725   $197.57   $197.55   $197.60  
  Purchase #3 $164.39   5.565%   $173.5383   $173.54   $173.55   $173.50  
  Purchase #4 $81.96   9.557%   $89.7929   $89.79   $89.80   $89.80  
  Purchase #5 $112.40   3.660%   $116.5138   $116.51   $116.50   $116.50  
  Purchase #6 $23.26   10.827%   $25.7784   $25.78   $25.80   $25.80  
  Purchase #7 $185.94   12.699%   $209.5525   $209.55   $209.55   $209.60  
  7 purchase totals = $772.79       $843.5339   $843.53   $843.55   $843.60