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 $70.76   11.719%   $79.0524   $79.05   $79.05   $79.10  
  Purchase #2 $248.28   9.691%   $272.3408   $272.34   $272.35   $272.30  
  Purchase #3 $26.29   4.420%   $27.452   $27.45   $27.45   $27.50  
  Purchase #4 $74.98   5.462%   $79.0754   $79.08   $79.10   $79.10  
  Purchase #5 $77.28   13.292%   $87.5521   $87.55   $87.55   $87.60  
  Purchase #6 $249.79   7.270%   $267.9497   $267.95   $267.95   $267.90  
  Purchase #7 $33.39   12.341%   $37.5107   $37.51   $37.50   $37.50  
  7 purchase totals = $780.77       $850.9331   $850.93   $850.95   $851.00