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 $229.30   8.151%   $247.9902   $247.99   $248.00   $248.00  
  Purchase #2 $105.86   7.659%   $113.9678   $113.97   $113.95   $114.00  
  Purchase #3 $76.45   10.633%   $84.5789   $84.58   $84.60   $84.60  
  Purchase #4 $194.41   11.579%   $216.9207   $216.92   $216.90   $216.90  
  Purchase #5 $95.68   8.717%   $104.0204   $104.02   $104.00   $104.00  
  Purchase #6 $77.79   10.172%   $85.7028   $85.70   $85.70   $85.70  
  Purchase #7 $58.42   12.158%   $65.5227   $65.52   $65.50   $65.50  
  7 purchase totals = $837.91       $918.7035   $918.70   $918.65   $918.70