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 $137.91   9.398%   $150.8708   $150.87   $150.85   $150.90  
  Purchase #2 $99.41   7.518%   $106.8836   $106.88   $106.90   $106.90  
  Purchase #3 $109.31   4.618%   $114.3579   $114.36   $114.35   $114.40  
  Purchase #4 $226.36   11.574%   $252.5589   $252.56   $252.55   $252.60  
  Purchase #5 $48.90   6.734%   $52.1929   $52.19   $52.20   $52.20  
  Purchase #6 $171.96   8.448%   $186.4872   $186.49   $186.50   $186.50  
  Purchase #7 $163.91   7.202%   $175.7148   $175.71   $175.70   $175.70  
  7 purchase totals = $957.76       $1,039.0661   $1,039.06   $1,039.05   $1,039.20