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 $224.84   12.293%   $252.4796   $252.48   $252.50   $252.50  
  Purchase #2 $203.30   5.666%   $214.819   $214.82   $214.80   $214.80  
  Purchase #3 $157.49   11.132%   $175.0218   $175.02   $175.00   $175.00  
  Purchase #4 $120.54   12.178%   $135.2194   $135.22   $135.20   $135.20  
  Purchase #5 $87.60   13.376%   $99.3174   $99.32   $99.30   $99.30  
  Purchase #6 $154.92   10.595%   $171.3338   $171.33   $171.35   $171.30  
  Purchase #7 $168.81   12.473%   $189.8657   $189.87   $189.85   $189.90  
  7 purchase totals = $1,117.50       $1,238.0567   $1,238.06   $1,238.00   $1,238.00