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 $21.31   12.844%   $24.0471   $24.05   $24.05   $24.00  
  Purchase #2 $193.67   13.270%   $219.37   $219.37   $219.35   $219.40  
  Purchase #3 $42.76   6.621%   $45.5911   $45.59   $45.60   $45.60  
  Purchase #4 $6.48   8.269%   $7.0158   $7.02   $7.00   $7.00  
  Purchase #5 $40.91   4.232%   $42.6413   $42.64   $42.65   $42.60  
  Purchase #6 $18.53   9.283%   $20.2501   $20.25   $20.25   $20.30  
  Purchase #7 $202.84   4.464%   $211.8948   $211.89   $211.90   $211.90  
  7 purchase totals = $526.50       $570.8102   $570.81   $570.80   $570.80