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 $248.83   8.342%   $269.5874   $269.59   $269.60   $269.60  
  Purchase #2 $10.50   13.217%   $11.8878   $11.89   $11.90   $11.90  
  Purchase #3 $52.51   6.258%   $55.7961   $55.80   $55.80   $55.80  
  Purchase #4 $127.29   6.178%   $135.154   $135.15   $135.15   $135.20  
  Purchase #5 $201.55   9.461%   $220.6186   $220.62   $220.60   $220.60  
  Purchase #6 $61.35   5.488%   $64.7169   $64.72   $64.70   $64.70  
  Purchase #7 $142.35   9.202%   $155.449   $155.45   $155.45   $155.40  
  7 purchase totals = $844.38       $913.2098   $913.22   $913.20   $913.20