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 $28.91   11.851%   $32.3361   $32.34   $32.35   $32.30  
  Purchase #2 $225.56   7.225%   $241.8567   $241.86   $241.85   $241.90  
  Purchase #3 $88.58   9.425%   $96.9287   $96.93   $96.95   $96.90  
  Purchase #4 $226.74   4.649%   $237.2811   $237.28   $237.30   $237.30  
  Purchase #5 $162.44   3.566%   $168.2326   $168.23   $168.25   $168.20  
  Purchase #6 $210.51   6.768%   $224.7573   $224.76   $224.75   $224.80  
  Purchase #7 $16.91   12.464%   $19.0177   $19.02   $19.00   $19.00  
  7 purchase totals = $959.65       $1,020.4102   $1,020.42   $1,020.45   $1,020.40