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 $249.54   13.633%   $283.5598   $283.56   $283.55   $283.60  
  Purchase #2 $180.32   7.748%   $194.2912   $194.29   $194.30   $194.30  
  Purchase #3 $201.60   9.393%   $220.5363   $220.54   $220.55   $220.50  
  Purchase #4 $76.69   10.448%   $84.7026   $84.70   $84.70   $84.70  
  Purchase #5 $95.56   6.483%   $101.7552   $101.76   $101.75   $101.80  
  Purchase #6 $56.75   12.554%   $63.8744   $63.87   $63.85   $63.90  
  Purchase #7 $18.33   8.491%   $19.8864   $19.89   $19.90   $19.90  
  7 purchase totals = $878.79       $968.6059   $968.61   $968.60   $968.70