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 $228.64   12.684%   $257.6407   $257.64   $257.65   $257.60  
  Purchase #2 $241.80   6.688%   $257.9716   $257.97   $257.95   $258.00  
  Purchase #3 $59.46   9.147%   $64.8988   $64.90   $64.90   $64.90  
  Purchase #4 $32.66   6.160%   $34.6719   $34.67   $34.65   $34.70  
  Purchase #5 $98.82   7.698%   $106.4272   $106.43   $106.45   $106.40  
  Purchase #6 $203.39   11.654%   $227.0931   $227.09   $227.10   $227.10  
  Purchase #7 $49.53   9.513%   $54.2418   $54.24   $54.25   $54.20  
  7 purchase totals = $914.30       $1,002.9451   $1,002.94   $1,002.95   $1,002.90