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 $245.87   13.739%   $279.6501   $279.65   $279.65   $279.70  
  Purchase #2 $111.51   11.485%   $124.3169   $124.32   $124.30   $124.30  
  Purchase #3 $232.82   8.594%   $252.8286   $252.83   $252.85   $252.80  
  Purchase #4 $157.54   6.333%   $167.517   $167.52   $167.50   $167.50  
  Purchase #5 $211.48   11.426%   $235.6437   $235.64   $235.65   $235.60  
  Purchase #6 $167.87   13.145%   $189.9365   $189.94   $189.95   $189.90  
  Purchase #7 $242.46   11.271%   $269.7877   $269.79   $269.80   $269.80  
  7 purchase totals = $1,369.55       $1,519.6805   $1,519.69   $1,519.70   $1,519.60