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 $241.67   11.489%   $269.4355   $269.44   $269.45   $269.40  
  Purchase #2 $103.33   7.862%   $111.4538   $111.45   $111.45   $111.50  
  Purchase #3 $15.95   5.305%   $16.7961   $16.80   $16.80   $16.80  
  Purchase #4 $53.59   12.519%   $60.2989   $60.30   $60.30   $60.30  
  Purchase #5 $216.89   13.107%   $245.3178   $245.32   $245.30   $245.30  
  Purchase #6 $147.94   12.382%   $166.2579   $166.26   $166.25   $166.30  
  Purchase #7 $114.66   5.758%   $121.2621   $121.26   $121.25   $121.30  
  7 purchase totals = $894.03       $990.8221   $990.83   $990.80   $990.90