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 $18.99   7.514%   $20.4169   $20.42   $20.40   $20.40  
  Purchase #2 $22.67   13.464%   $25.7223   $25.72   $25.70   $25.70  
  Purchase #3 $86.28   12.458%   $97.0288   $97.03   $97.05   $97.00  
  Purchase #4 $137.47   11.592%   $153.4055   $153.41   $153.40   $153.40  
  Purchase #5 $154.65   3.264%   $159.6978   $159.70   $159.70   $159.70  
  Purchase #6 $160.38   3.831%   $166.5242   $166.52   $166.50   $166.50  
  Purchase #7 $123.99   3.682%   $128.5553   $128.56   $128.55   $128.60  
  7 purchase totals = $704.43       $751.3508   $751.36   $751.30   $751.30