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 $18.25   13.638%   $20.7389   $20.74   $20.75   $20.70  
  Purchase #2 $12.60   3.621%   $13.0562   $13.06   $13.05   $13.10  
  Purchase #3 $222.36   7.598%   $239.2549   $239.25   $239.25   $239.30  
  Purchase #4 $203.49   12.842%   $229.6222   $229.62   $229.60   $229.60  
  Purchase #5 $98.59   12.692%   $111.103   $111.10   $111.10   $111.10  
  Purchase #6 $229.66   12.518%   $258.4088   $258.41   $258.40   $258.40  
  Purchase #7 $225.48   7.163%   $241.6311   $241.63   $241.65   $241.60  
  7 purchase totals = $1,010.43       $1,113.8151   $1,113.81   $1,113.80   $1,113.80