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 $19.86   10.136%   $21.873   $21.87   $21.85   $21.90  
  Purchase #2 $19.88   9.692%   $21.8068   $21.81   $21.80   $21.80  
  Purchase #3 $199.99   4.296%   $208.5816   $208.58   $208.60   $208.60  
  Purchase #4 $189.80   8.391%   $205.7261   $205.73   $205.75   $205.70  
  Purchase #5 $36.81   11.606%   $41.0822   $41.08   $41.10   $41.10  
  Purchase #6 $202.26   6.260%   $214.9215   $214.92   $214.90   $214.90  
  Purchase #7 $63.71   6.512%   $67.8588   $67.86   $67.85   $67.90  
  7 purchase totals = $732.31       $781.8500   $781.85   $781.85   $781.90