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 $29.45   4.221%   $30.6931   $30.69   $30.70   $30.70  
  Purchase #2 $75.85   10.795%   $84.038   $84.04   $84.05   $84.00  
  Purchase #3 $22.31   10.378%   $24.6253   $24.63   $24.65   $24.60  
  Purchase #4 $90.27   4.648%   $94.4657   $94.47   $94.45   $94.50  
  Purchase #5 $22.70   6.120%   $24.0892   $24.09   $24.10   $24.10  
  Purchase #6 $207.70   5.782%   $219.7092   $219.71   $219.70   $219.70  
  Purchase #7 $82.68   11.819%   $92.4519   $92.45   $92.45   $92.50  
  7 purchase totals = $530.96       $570.0724   $570.08   $570.10   $570.10