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 $223.66   12.463%   $251.5347   $251.53   $251.55   $251.50  
  Purchase #2 $153.86   4.588%   $160.9191   $160.92   $160.90   $160.90  
  Purchase #3 $62.69   8.796%   $68.2042   $68.20   $68.20   $68.20  
  Purchase #4 $150.54   5.352%   $158.5969   $158.60   $158.60   $158.60  
  Purchase #5 $82.97   10.159%   $91.3989   $91.40   $91.40   $91.40  
  Purchase #6 $47.58   4.457%   $49.7006   $49.70   $49.70   $49.70  
  Purchase #7 $6.31   8.583%   $6.8516   $6.85   $6.85   $6.90  
  7 purchase totals = $727.61       $787.2060   $787.20   $787.20   $787.20