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 $8.88   10.605%   $9.8217   $9.82   $9.80   $9.80  
  Purchase #2 $11.41   5.768%   $12.0681   $12.07   $12.05   $12.10  
  Purchase #3 $144.95   6.524%   $154.4065   $154.41   $154.40   $154.40  
  Purchase #4 $184.69   8.528%   $200.4404   $200.44   $200.45   $200.40  
  Purchase #5 $83.29   6.174%   $88.4323   $88.43   $88.45   $88.40  
  Purchase #6 $39.95   13.112%   $45.1882   $45.19   $45.20   $45.20  
  Purchase #7 $83.65   8.117%   $90.4399   $90.44   $90.45   $90.40  
  7 purchase totals = $556.82       $600.7971   $600.80   $600.80   $600.70