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 $229.63   8.540%   $249.2404   $249.24   $249.25   $249.20  
  Purchase #2 $79.84   10.179%   $87.9669   $87.97   $87.95   $88.00  
  Purchase #3 $212.72   3.723%   $220.6396   $220.64   $220.65   $220.60  
  Purchase #4 $79.74   9.389%   $87.2268   $87.23   $87.25   $87.20  
  Purchase #5 $93.89   7.119%   $100.574   $100.57   $100.55   $100.60  
  Purchase #6 $159.72   9.519%   $174.9237   $174.92   $174.90   $174.90  
  Purchase #7 $123.57   8.185%   $133.6842   $133.68   $133.70   $133.70  
  7 purchase totals = $979.11       $1,054.2556   $1,054.25   $1,054.25   $1,054.20