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 $246.97   13.352%   $279.9454   $279.95   $279.95   $279.90  
  Purchase #2 $167.42   11.684%   $186.9814   $186.98   $187.00   $187.00  
  Purchase #3 $59.77   12.619%   $67.3124   $67.31   $67.30   $67.30  
  Purchase #4 $241.63   12.839%   $272.6529   $272.65   $272.65   $272.70  
  Purchase #5 $70.55   8.519%   $76.5602   $76.56   $76.55   $76.60  
  Purchase #6 $114.32   7.627%   $123.0392   $123.04   $123.05   $123.00  
  Purchase #7 $90.72   4.468%   $94.7734   $94.77   $94.75   $94.80  
  7 purchase totals = $991.38       $1,101.2649   $1,101.26   $1,101.25   $1,101.30