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 $35.45   12.514%   $39.8862   $39.89   $39.90   $39.90  
  Purchase #2 $182.56   5.680%   $192.9294   $192.93   $192.95   $192.90  
  Purchase #3 $139.36   13.780%   $158.5638   $158.56   $158.55   $158.60  
  Purchase #4 $189.79   8.738%   $206.3739   $206.37   $206.35   $206.40  
  Purchase #5 $130.34   5.584%   $137.6182   $137.62   $137.60   $137.60  
  Purchase #6 $69.69   7.104%   $74.6408   $74.64   $74.65   $74.60  
  Purchase #7 $87.43   4.356%   $91.2385   $91.24   $91.25   $91.20  
  7 purchase totals = $834.62       $901.2508   $901.25   $901.25   $901.20