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 $38.90   3.380%   $40.2148   $40.21   $40.20   $40.20  
  Purchase #2 $216.90   6.182%   $230.3088   $230.31   $230.30   $230.30  
  Purchase #3 $31.59   7.163%   $33.8528   $33.85   $33.85   $33.90  
  Purchase #4 $238.92   10.424%   $263.825   $263.83   $263.85   $263.80  
  Purchase #5 $103.64   6.475%   $110.3507   $110.35   $110.35   $110.40  
  Purchase #6 $21.80   5.848%   $23.0749   $23.07   $23.05   $23.10  
  Purchase #7 $125.83   13.579%   $142.9165   $142.92   $142.90   $142.90  
  7 purchase totals = $777.58       $844.5435   $844.54   $844.50   $844.60