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 $15.79   3.599%   $16.3583   $16.36   $16.35   $16.40  
  Purchase #2 $45.29   8.353%   $49.0731   $49.07   $49.05   $49.10  
  Purchase #3 $193.61   8.682%   $210.4192   $210.42   $210.40   $210.40  
  Purchase #4 $137.30   9.385%   $150.1856   $150.19   $150.20   $150.20  
  Purchase #5 $162.69   12.308%   $182.7139   $182.71   $182.70   $182.70  
  Purchase #6 $90.72   7.469%   $97.4959   $97.50   $97.50   $97.50  
  Purchase #7 $122.33   10.404%   $135.0572   $135.06   $135.05   $135.10  
  7 purchase totals = $767.73       $841.3032   $841.31   $841.25   $841.40