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 $23.61   12.114%   $26.4701   $26.47   $26.45   $26.50  
  Purchase #2 $164.55   13.489%   $186.7461   $186.75   $186.75   $186.70  
  Purchase #3 $233.29   7.184%   $250.0496   $250.05   $250.05   $250.00  
  Purchase #4 $216.92   11.816%   $242.5513   $242.55   $242.55   $242.60  
  Purchase #5 $32.82   10.631%   $36.3091   $36.31   $36.30   $36.30  
  Purchase #6 $72.78   5.252%   $76.6024   $76.60   $76.60   $76.60  
  Purchase #7 $26.51   8.832%   $28.8514   $28.85   $28.85   $28.90  
  7 purchase totals = $770.48       $847.5800   $847.58   $847.55   $847.60