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 $246.77   9.106%   $269.2409   $269.24   $269.25   $269.20  
  Purchase #2 $53.72   6.780%   $57.3622   $57.36   $57.35   $57.40  
  Purchase #3 $9.40   4.736%   $9.8452   $9.85   $9.85   $9.80  
  Purchase #4 $67.45   5.598%   $71.2259   $71.23   $71.25   $71.20  
  Purchase #5 $102.42   11.580%   $114.2802   $114.28   $114.30   $114.30  
  Purchase #6 $227.38   13.388%   $257.8216   $257.82   $257.80   $257.80  
  Purchase #7 $35.33   3.146%   $36.4415   $36.44   $36.45   $36.40  
  7 purchase totals = $742.47       $816.2175   $816.22   $816.25   $816.10