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 $241.30   3.538%   $249.8372   $249.84   $249.85   $249.80  
  Purchase #2 $152.93   11.455%   $170.4481   $170.45   $170.45   $170.40  
  Purchase #3 $24.61   10.113%   $27.0988   $27.10   $27.10   $27.10  
  Purchase #4 $106.57   10.112%   $117.3464   $117.35   $117.35   $117.30  
  Purchase #5 $101.31   7.368%   $108.7745   $108.77   $108.75   $108.80  
  Purchase #6 $170.44   13.780%   $193.9266   $193.93   $193.95   $193.90  
  Purchase #7 $197.79   11.185%   $219.9128   $219.91   $219.90   $219.90  
  7 purchase totals = $994.95       $1,087.3444   $1,087.35   $1,087.35   $1,087.20