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 $240.87   13.348%   $273.0213   $273.02   $273.00   $273.00  
  Purchase #2 $101.95   10.184%   $112.3326   $112.33   $112.35   $112.30  
  Purchase #3 $56.68   6.735%   $60.4974   $60.50   $60.50   $60.50  
  Purchase #4 $106.35   4.333%   $110.9581   $110.96   $110.95   $111.00  
  Purchase #5 $211.62   3.726%   $219.505   $219.51   $219.50   $219.50  
  Purchase #6 $131.88   3.526%   $136.5301   $136.53   $136.55   $136.50  
  Purchase #7 $29.77   9.671%   $32.6491   $32.65   $32.65   $32.60  
  7 purchase totals = $879.12       $945.4936   $945.50   $945.50   $945.40