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 $243.36   12.172%   $272.9818   $272.98   $273.00   $273.00  
  Purchase #2 $101.82   8.668%   $110.6458   $110.65   $110.65   $110.60  
  Purchase #3 $98.71   13.143%   $111.6835   $111.68   $111.70   $111.70  
  Purchase #4 $83.60   13.834%   $95.1652   $95.17   $95.15   $95.20  
  Purchase #5 $145.67   4.482%   $152.1989   $152.20   $152.20   $152.20  
  Purchase #6 $231.28   3.328%   $238.977   $238.98   $239.00   $239.00  
  Purchase #7 $53.51   12.349%   $60.1179   $60.12   $60.10   $60.10  
  7 purchase totals = $957.95       $1,041.7701   $1,041.78   $1,041.80   $1,041.80