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 $120.51   12.671%   $135.7798   $135.78   $135.80   $135.80  
  Purchase #2 $200.48   9.410%   $219.3452   $219.35   $219.35   $219.30  
  Purchase #3 $228.45   9.847%   $250.9455   $250.95   $250.95   $250.90  
  Purchase #4 $16.63   5.427%   $17.5325   $17.53   $17.55   $17.50  
  Purchase #5 $107.99   13.430%   $122.4931   $122.49   $122.50   $122.50  
  Purchase #6 $133.68   6.670%   $142.5965   $142.60   $142.60   $142.60  
  Purchase #7 $152.75   6.825%   $163.1752   $163.18   $163.20   $163.20  
  7 purchase totals = $960.49       $1,051.8678   $1,051.88   $1,051.95   $1,051.80