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 $18.26   11.615%   $20.3809   $20.38   $20.40   $20.40  
  Purchase #2 $211.50   3.510%   $218.9237   $218.92   $218.90   $218.90  
  Purchase #3 $5.89   7.258%   $6.3175   $6.32   $6.30   $6.30  
  Purchase #4 $121.41   3.109%   $125.1846   $125.18   $125.20   $125.20  
  Purchase #5 $178.65   13.429%   $202.6409   $202.64   $202.65   $202.60  
  Purchase #6 $219.50   9.730%   $240.8574   $240.86   $240.85   $240.90  
  Purchase #7 $159.71   12.116%   $179.0605   $179.06   $179.05   $179.10  
  7 purchase totals = $914.92       $993.3655   $993.36   $993.35   $993.40