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 $85.90   4.749%   $89.9794   $89.98   $90.00   $90.00  
  Purchase #2 $69.61   8.474%   $75.5088   $75.51   $75.50   $75.50  
  Purchase #3 $78.57   4.611%   $82.1929   $82.19   $82.20   $82.20  
  Purchase #4 $24.68   13.639%   $28.0461   $28.05   $28.05   $28.00  
  Purchase #5 $48.97   12.561%   $55.1211   $55.12   $55.10   $55.10  
  Purchase #6 $173.82   8.610%   $188.7859   $188.79   $188.80   $188.80  
  Purchase #7 $104.68   9.659%   $114.791   $114.79   $114.80   $114.80  
  7 purchase totals = $586.23       $634.4252   $634.43   $634.45   $634.40