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 $27.93   12.627%   $31.4567   $31.46   $31.45   $31.50  
  Purchase #2 $189.43   10.150%   $208.6571   $208.66   $208.65   $208.70  
  Purchase #3 $188.33   9.734%   $206.662   $206.66   $206.65   $206.70  
  Purchase #4 $101.98   6.130%   $108.2314   $108.23   $108.25   $108.20  
  Purchase #5 $183.65   13.468%   $208.384   $208.38   $208.40   $208.40  
  Purchase #6 $102.96   10.704%   $113.9808   $113.98   $114.00   $114.00  
  Purchase #7 $116.75   6.103%   $123.8753   $123.88   $123.90   $123.90  
  7 purchase totals = $911.03       $1,001.2473   $1,001.25   $1,001.30   $1,001.40