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 $238.78   12.209%   $267.9327   $267.93   $267.95   $267.90  
  Purchase #2 $14.39   7.383%   $15.4524   $15.45   $15.45   $15.50  
  Purchase #3 $53.28   13.182%   $60.3034   $60.30   $60.30   $60.30  
  Purchase #4 $8.29   12.188%   $9.3004   $9.30   $9.30   $9.30  
  Purchase #5 $93.28   7.426%   $100.207   $100.21   $100.20   $100.20  
  Purchase #6 $171.26   4.759%   $179.4103   $179.41   $179.40   $179.40  
  Purchase #7 $155.60   3.612%   $161.2203   $161.22   $161.20   $161.20  
  7 purchase totals = $734.88       $793.8265   $793.82   $793.80   $793.80