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 $25.32   12.238%   $28.4187   $28.42   $28.40   $28.40  
  Purchase #2 $198.81   7.212%   $213.1482   $213.15   $213.15   $213.10  
  Purchase #3 $138.49   11.530%   $154.4579   $154.46   $154.45   $154.50  
  Purchase #4 $6.31   9.668%   $6.9201   $6.92   $6.90   $6.90  
  Purchase #5 $6.99   3.575%   $7.2399   $7.24   $7.25   $7.20  
  Purchase #6 $150.66   10.222%   $166.0605   $166.06   $166.05   $166.10  
  Purchase #7 $229.78   10.390%   $253.6541   $253.65   $253.65   $253.70  
  7 purchase totals = $756.36       $829.8994   $829.90   $829.85   $829.90