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 $15.76   6.580%   $16.797   $16.80   $16.80   $16.80  
  Purchase #2 $176.42   4.317%   $184.0361   $184.04   $184.05   $184.00  
  Purchase #3 $5.90   7.668%   $6.3524   $6.35   $6.35   $6.40  
  Purchase #4 $17.72   11.502%   $19.7582   $19.76   $19.75   $19.80  
  Purchase #5 $249.29   6.119%   $264.5441   $264.54   $264.55   $264.50  
  Purchase #6 $164.34   12.778%   $185.3394   $185.34   $185.35   $185.30  
  Purchase #7 $10.74   12.325%   $12.0637   $12.06   $12.05   $12.10  
  7 purchase totals = $640.17       $688.8909   $688.89   $688.90   $688.90