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 $238.92   4.633%   $249.9892   $249.99   $250.00   $250.00  
  Purchase #2 $5.98   8.541%   $6.4908   $6.49   $6.50   $6.50  
  Purchase #3 $155.31   13.667%   $176.5362   $176.54   $176.55   $176.50  
  Purchase #4 $25.83   12.641%   $29.0952   $29.10   $29.10   $29.10  
  Purchase #5 $211.62   8.847%   $230.342   $230.34   $230.35   $230.30  
  Purchase #6 $175.46   13.153%   $198.5383   $198.54   $198.55   $198.50  
  Purchase #7 $180.58   5.738%   $190.9417   $190.94   $190.95   $190.90  
  7 purchase totals = $993.70       $1,081.9334   $1,081.94   $1,082.00   $1,081.80