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 $26.29   12.718%   $29.6336   $29.63   $29.65   $29.60  
  Purchase #2 $103.45   6.738%   $110.4205   $110.42   $110.40   $110.40  
  Purchase #3 $107.88   11.524%   $120.3121   $120.31   $120.30   $120.30  
  Purchase #4 $166.76   12.235%   $187.1631   $187.16   $187.15   $187.20  
  Purchase #5 $94.57   4.749%   $99.0611   $99.06   $99.05   $99.10  
  Purchase #6 $62.51   9.302%   $68.3247   $68.32   $68.30   $68.30  
  Purchase #7 $61.94   13.558%   $70.3378   $70.34   $70.35   $70.30  
  7 purchase totals = $623.40       $685.2529   $685.24   $685.20   $685.20