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 $40.99   4.338%   $42.7681   $42.77   $42.75   $42.80  
  Purchase #2 $47.88   7.232%   $51.3427   $51.34   $51.35   $51.30  
  Purchase #3 $80.42   4.865%   $84.3324   $84.33   $84.35   $84.30  
  Purchase #4 $65.80   10.219%   $72.5241   $72.52   $72.50   $72.50  
  Purchase #5 $22.35   6.506%   $23.8041   $23.80   $23.80   $23.80  
  Purchase #6 $77.97   8.757%   $84.7978   $84.80   $84.80   $84.80  
  Purchase #7 $116.28   10.570%   $128.5708   $128.57   $128.55   $128.60  
  7 purchase totals = $451.69       $488.1400   $488.13   $488.10   $488.10