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 $249.93   3.372%   $258.3576   $258.36   $258.35   $258.40  
  Purchase #2 $224.47   13.768%   $255.375   $255.38   $255.40   $255.40  
  Purchase #3 $99.45   4.740%   $104.1639   $104.16   $104.15   $104.20  
  Purchase #4 $194.77   12.451%   $219.0208   $219.02   $219.00   $219.00  
  Purchase #5 $84.81   13.407%   $96.1805   $96.18   $96.20   $96.20  
  Purchase #6 $205.93   5.839%   $217.9543   $217.95   $217.95   $218.00  
  Purchase #7 $163.54   4.515%   $170.9238   $170.92   $170.90   $170.90  
  7 purchase totals = $1,222.90       $1,321.9759   $1,321.97   $1,321.95   $1,322.10