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 $250.84   8.522%   $272.2166   $272.22   $272.20   $272.20  
  Purchase #2 $10.33   11.421%   $11.5098   $11.51   $11.50   $11.50  
  Purchase #3 $90.39   8.231%   $97.83   $97.83   $97.85   $97.80  
  Purchase #4 $182.42   7.297%   $195.7312   $195.73   $195.75   $195.70  
  Purchase #5 $145.78   4.237%   $151.9567   $151.96   $151.95   $152.00  
  Purchase #6 $67.50   6.647%   $71.9867   $71.99   $72.00   $72.00  
  Purchase #7 $169.91   7.858%   $183.2615   $183.26   $183.25   $183.30  
  7 purchase totals = $917.17       $984.4925   $984.50   $984.50   $984.50