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 $179.47   13.563%   $203.8115   $203.81   $203.80   $203.80  
  Purchase #2 $178.92   11.336%   $199.2024   $199.20   $199.20   $199.20  
  Purchase #3 $145.50   13.404%   $165.0028   $165.00   $165.00   $165.00  
  Purchase #4 $216.70   9.576%   $237.4512   $237.45   $237.45   $237.50  
  Purchase #5 $154.91   8.829%   $168.587   $168.59   $168.60   $168.60  
  Purchase #6 $16.50   9.244%   $18.0253   $18.03   $18.05   $18.00  
  Purchase #7 $34.25   11.132%   $38.0627   $38.06   $38.05   $38.10  
  7 purchase totals = $926.25       $1,030.1429   $1,030.14   $1,030.15   $1,030.20