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 $246.49   12.210%   $276.5864   $276.59   $276.60   $276.60  
  Purchase #2 $201.89   6.723%   $215.4631   $215.46   $215.45   $215.50  
  Purchase #3 $156.80   5.301%   $165.112   $165.11   $165.10   $165.10  
  Purchase #4 $67.81   10.562%   $74.9721   $74.97   $74.95   $75.00  
  Purchase #5 $203.48   4.675%   $212.9927   $212.99   $213.00   $213.00  
  Purchase #6 $162.65   4.671%   $170.2474   $170.25   $170.25   $170.20  
  Purchase #7 $172.37   6.692%   $183.905   $183.91   $183.90   $183.90  
  7 purchase totals = $1,211.49       $1,299.2787   $1,299.28   $1,299.25   $1,299.30