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 $56.36   12.463%   $63.3841   $63.38   $63.40   $63.40  
  Purchase #2 $112.96   10.233%   $124.5192   $124.52   $124.50   $124.50  
  Purchase #3 $176.74   7.412%   $189.84   $189.84   $189.85   $189.80  
  Purchase #4 $94.83   4.243%   $98.8536   $98.85   $98.85   $98.90  
  Purchase #5 $116.45   13.385%   $132.0368   $132.04   $132.05   $132.00  
  Purchase #6 $182.40   7.203%   $195.5383   $195.54   $195.55   $195.50  
  Purchase #7 $52.82   5.618%   $55.7874   $55.79   $55.80   $55.80  
  7 purchase totals = $792.56       $859.9594   $859.96   $860.00   $859.90