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 $240.75   13.702%   $273.7376   $273.74   $273.75   $273.70  
  Purchase #2 $217.91   4.551%   $227.8271   $227.83   $227.85   $227.80  
  Purchase #3 $64.75   9.473%   $70.8838   $70.88   $70.90   $70.90  
  Purchase #4 $60.55   13.519%   $68.7358   $68.74   $68.75   $68.70  
  Purchase #5 $233.26   8.226%   $252.448   $252.45   $252.45   $252.40  
  Purchase #6 $103.78   8.211%   $112.3014   $112.30   $112.30   $112.30  
  Purchase #7 $149.84   3.165%   $154.5824   $154.58   $154.60   $154.60  
  7 purchase totals = $1,070.84       $1,160.5161   $1,160.52   $1,160.60   $1,160.40