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 $21.35   8.239%   $23.109   $23.11   $23.10   $23.10  
  Purchase #2 $26.66   7.139%   $28.5633   $28.56   $28.55   $28.60  
  Purchase #3 $204.25   10.561%   $225.8208   $225.82   $225.80   $225.80  
  Purchase #4 $12.41   4.620%   $12.9833   $12.98   $13.00   $13.00  
  Purchase #5 $5.87   11.270%   $6.5315   $6.53   $6.55   $6.50  
  Purchase #6 $162.48   10.807%   $180.0392   $180.04   $180.05   $180.00  
  Purchase #7 $9.75   6.309%   $10.3651   $10.37   $10.35   $10.40  
  7 purchase totals = $442.77       $487.4122   $487.41   $487.40   $487.40