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 $250.56   13.493%   $284.3681   $284.37   $284.35   $284.40  
  Purchase #2 $58.79   13.580%   $66.7737   $66.77   $66.75   $66.80  
  Purchase #3 $95.35   12.488%   $107.2573   $107.26   $107.25   $107.30  
  Purchase #4 $68.58   12.562%   $77.195   $77.20   $77.20   $77.20  
  Purchase #5 $232.92   7.543%   $250.4892   $250.49   $250.50   $250.50  
  Purchase #6 $86.72   8.867%   $94.4095   $94.41   $94.40   $94.40  
  Purchase #7 $162.52   12.821%   $183.3567   $183.36   $183.35   $183.40  
  7 purchase totals = $955.44       $1,063.8495   $1,063.86   $1,063.80   $1,064.00