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 $57.57   12.510%   $64.772   $64.77   $64.75   $64.80  
  Purchase #2 $131.38   6.672%   $140.1457   $140.15   $140.15   $140.10  
  Purchase #3 $41.81   5.343%   $44.0439   $44.04   $44.05   $44.00  
  Purchase #4 $215.72   5.610%   $227.8219   $227.82   $227.80   $227.80  
  Purchase #5 $26.96   12.193%   $30.2472   $30.25   $30.25   $30.20  
  Purchase #6 $129.76   13.437%   $147.1959   $147.20   $147.20   $147.20  
  Purchase #7 $242.50   9.234%   $264.8925   $264.89   $264.90   $264.90  
  7 purchase totals = $845.70       $919.1191   $919.12   $919.10   $919.00