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 $245.58   10.501%   $271.3684   $271.37   $271.35   $271.40  
  Purchase #2 $47.37   3.213%   $48.892   $48.89   $48.90   $48.90  
  Purchase #3 $117.37   5.508%   $123.8347   $123.83   $123.85   $123.80  
  Purchase #4 $248.89   13.792%   $283.2169   $283.22   $283.20   $283.20  
  Purchase #5 $81.55   8.282%   $88.304   $88.30   $88.30   $88.30  
  Purchase #6 $82.74   12.553%   $93.1264   $93.13   $93.15   $93.10  
  Purchase #7 $59.99   8.796%   $65.2667   $65.27   $65.25   $65.30  
  7 purchase totals = $883.49       $974.0091   $974.01   $974.00   $974.00