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 $103.72   4.289%   $108.1686   $108.17   $108.15   $108.20  
  Purchase #2 $68.91   7.776%   $74.2684   $74.27   $74.25   $74.30  
  Purchase #3 $38.59   8.140%   $41.7312   $41.73   $41.75   $41.70  
  Purchase #4 $108.39   10.666%   $119.9509   $119.95   $119.95   $120.00  
  Purchase #5 $79.39   12.181%   $89.0605   $89.06   $89.05   $89.10  
  Purchase #6 $93.48   7.394%   $100.3919   $100.39   $100.40   $100.40  
  Purchase #7 $98.59   12.223%   $110.6407   $110.64   $110.65   $110.60  
  7 purchase totals = $591.07       $644.2122   $644.21   $644.20   $644.30