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 $234.77   6.237%   $249.4126   $249.41   $249.40   $249.40  
  Purchase #2 $71.76   7.768%   $77.3343   $77.33   $77.35   $77.30  
  Purchase #3 $91.56   3.498%   $94.7628   $94.76   $94.75   $94.80  
  Purchase #4 $242.27   5.229%   $254.9383   $254.94   $254.95   $254.90  
  Purchase #5 $23.76   5.198%   $24.995   $25.00   $25.00   $25.00  
  Purchase #6 $29.80   4.262%   $31.0701   $31.07   $31.05   $31.10  
  Purchase #7 $60.99   6.160%   $64.747   $64.75   $64.75   $64.70  
  7 purchase totals = $754.91       $797.2601   $797.26   $797.25   $797.20