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 $12.31   7.481%   $13.2309   $13.23   $13.25   $13.20  
  Purchase #2 $41.65   10.144%   $45.875   $45.88   $45.90   $45.90  
  Purchase #3 $227.40   11.536%   $253.6329   $253.63   $253.65   $253.60  
  Purchase #4 $6.64   10.685%   $7.3495   $7.35   $7.35   $7.30  
  Purchase #5 $109.93   5.382%   $115.8464   $115.85   $115.85   $115.80  
  Purchase #6 $59.30   9.413%   $64.8819   $64.88   $64.90   $64.90  
  Purchase #7 $87.37   8.323%   $94.6418   $94.64   $94.65   $94.60  
  7 purchase totals = $544.60       $595.4584   $595.46   $595.55   $595.30