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 $248.87   12.153%   $279.1152   $279.12   $279.10   $279.10  
  Purchase #2 $225.95   8.368%   $244.8575   $244.86   $244.85   $244.90  
  Purchase #3 $125.96   4.538%   $131.6761   $131.68   $131.70   $131.70  
  Purchase #4 $111.52   13.666%   $126.7603   $126.76   $126.75   $126.80  
  Purchase #5 $116.75   4.759%   $122.3061   $122.31   $122.30   $122.30  
  Purchase #6 $37.43   13.670%   $42.5467   $42.55   $42.55   $42.50  
  Purchase #7 $217.35   4.336%   $226.7743   $226.77   $226.75   $226.80  
  7 purchase totals = $1,083.83       $1,174.0362   $1,174.05   $1,174.00   $1,174.10