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 $245.56   12.372%   $275.9407   $275.94   $275.95   $275.90  
  Purchase #2 $193.85   5.327%   $204.1764   $204.18   $204.20   $204.20  
  Purchase #3 $176.37   6.244%   $187.3825   $187.38   $187.40   $187.40  
  Purchase #4 $52.50   7.242%   $56.3021   $56.30   $56.30   $56.30  
  Purchase #5 $179.51   10.763%   $198.8307   $198.83   $198.85   $198.80  
  Purchase #6 $184.63   5.514%   $194.8105   $194.81   $194.80   $194.80  
  Purchase #7 $240.65   3.392%   $248.8128   $248.81   $248.80   $248.80  
  7 purchase totals = $1,273.07       $1,366.2557   $1,366.25   $1,366.30   $1,366.20