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 $239.33   13.261%   $271.0676   $271.07   $271.05   $271.10  
  Purchase #2 $94.45   6.371%   $100.4674   $100.47   $100.45   $100.50  
  Purchase #3 $124.51   5.481%   $131.3344   $131.33   $131.35   $131.30  
  Purchase #4 $130.63   6.608%   $139.262   $139.26   $139.25   $139.30  
  Purchase #5 $75.49   9.756%   $82.8548   $82.85   $82.85   $82.90  
  Purchase #6 $232.66   8.294%   $251.9568   $251.96   $251.95   $252.00  
  Purchase #7 $72.39   4.772%   $75.8445   $75.84   $75.85   $75.80  
  7 purchase totals = $969.46       $1,052.7875   $1,052.78   $1,052.75   $1,052.90