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 $77.42   4.511%   $80.9124   $80.91   $80.90   $80.90  
  Purchase #2 $8.45   4.172%   $8.8025   $8.80   $8.80   $8.80  
  Purchase #3 $14.89   9.780%   $16.3462   $16.35   $16.35   $16.30  
  Purchase #4 $170.76   9.474%   $186.9378   $186.94   $186.95   $186.90  
  Purchase #5 $24.44   3.847%   $25.3802   $25.38   $25.40   $25.40  
  Purchase #6 $133.39   10.576%   $147.4973   $147.50   $147.50   $147.50  
  Purchase #7 $185.73   12.300%   $208.5748   $208.57   $208.55   $208.60  
  7 purchase totals = $615.08       $674.4512   $674.45   $674.45   $674.40