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 $18.37   9.406%   $20.0979   $20.10   $20.10   $20.10  
  Purchase #2 $229.50   13.824%   $261.2261   $261.23   $261.25   $261.20  
  Purchase #3 $52.53   10.607%   $58.1019   $58.10   $58.10   $58.10  
  Purchase #4 $164.53   9.376%   $179.9563   $179.96   $179.95   $180.00  
  Purchase #5 $161.46   6.262%   $171.5706   $171.57   $171.55   $171.60  
  Purchase #6 $177.55   4.670%   $185.8416   $185.84   $185.85   $185.80  
  Purchase #7 $39.71   4.196%   $41.3762   $41.38   $41.40   $41.40  
  7 purchase totals = $843.65       $918.1706   $918.18   $918.20   $918.20