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 $35.84   13.440%   $40.6569   $40.66   $40.65   $40.70  
  Purchase #2 $139.32   7.752%   $150.1201   $150.12   $150.10   $150.10  
  Purchase #3 $187.45   3.719%   $194.4213   $194.42   $194.40   $194.40  
  Purchase #4 $147.54   9.457%   $161.4929   $161.49   $161.50   $161.50  
  Purchase #5 $103.73   4.439%   $108.3346   $108.33   $108.35   $108.30  
  Purchase #6 $183.48   4.338%   $191.4394   $191.44   $191.45   $191.40  
  Purchase #7 $157.85   12.429%   $177.4692   $177.47   $177.45   $177.50  
  7 purchase totals = $955.21       $1,023.9344   $1,023.93   $1,023.90   $1,023.90