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 $77.74   4.363%   $81.1318   $81.13   $81.15   $81.10  
  Purchase #2 $16.98   5.457%   $17.9066   $17.91   $17.90   $17.90  
  Purchase #3 $164.62   8.565%   $178.7197   $178.72   $178.70   $178.70  
  Purchase #4 $144.90   5.863%   $153.3955   $153.40   $153.40   $153.40  
  Purchase #5 $168.99   4.370%   $176.3749   $176.37   $176.35   $176.40  
  Purchase #6 $38.75   13.327%   $43.9142   $43.91   $43.90   $43.90  
  Purchase #7 $5.42   8.566%   $5.8843   $5.88   $5.90   $5.90  
  7 purchase totals = $617.40       $657.3270   $657.32   $657.30   $657.30