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 $175.44   12.356%   $197.1174   $197.12   $197.10   $197.10  
  Purchase #2 $242.71   6.710%   $258.9958   $259.00   $259.00   $259.00  
  Purchase #3 $114.90   11.831%   $128.4938   $128.49   $128.50   $128.50  
  Purchase #4 $52.91   8.418%   $57.364   $57.36   $57.35   $57.40  
  Purchase #5 $165.55   13.397%   $187.7287   $187.73   $187.75   $187.70  
  Purchase #6 $48.32   12.196%   $54.2131   $54.21   $54.20   $54.20  
  Purchase #7 $29.85   5.184%   $31.3974   $31.40   $31.40   $31.40  
  7 purchase totals = $829.68       $915.3102   $915.31   $915.30   $915.30