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 $15.57   11.505%   $17.3613   $17.36   $17.35   $17.40  
  Purchase #2 $5.28   9.207%   $5.7661   $5.77   $5.75   $5.80  
  Purchase #3 $207.55   5.800%   $219.5879   $219.59   $219.60   $219.60  
  Purchase #4 $6.57   13.335%   $7.4461   $7.45   $7.45   $7.40  
  Purchase #5 $197.40   8.140%   $213.4684   $213.47   $213.45   $213.50  
  Purchase #6 $77.38   11.695%   $86.4296   $86.43   $86.45   $86.40  
  Purchase #7 $36.88   10.668%   $40.8144   $40.81   $40.80   $40.80  
  7 purchase totals = $546.63       $590.8738   $590.88   $590.85   $590.90