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 $179.54   13.261%   $203.3488   $203.35   $203.35   $203.30  
  Purchase #2 $37.85   8.799%   $41.1804   $41.18   $41.20   $41.20  
  Purchase #3 $227.86   4.667%   $238.4942   $238.49   $238.50   $238.50  
  Purchase #4 $198.76   11.144%   $220.9098   $220.91   $220.90   $220.90  
  Purchase #5 $44.99   8.757%   $48.9298   $48.93   $48.95   $48.90  
  Purchase #6 $56.91   10.246%   $62.741   $62.74   $62.75   $62.70  
  Purchase #7 $23.63   12.148%   $26.5006   $26.50   $26.50   $26.50  
  7 purchase totals = $769.54       $842.1046   $842.10   $842.15   $842.00