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 $248.57   3.544%   $257.3793   $257.38   $257.40   $257.40  
  Purchase #2 $129.26   8.268%   $139.9472   $139.95   $139.95   $139.90  
  Purchase #3 $104.60   11.823%   $116.9669   $116.97   $116.95   $117.00  
  Purchase #4 $247.61   6.605%   $263.9646   $263.96   $263.95   $264.00  
  Purchase #5 $203.78   4.495%   $212.9399   $212.94   $212.95   $212.90  
  Purchase #6 $222.78   11.472%   $248.3373   $248.34   $248.35   $248.30  
  Purchase #7 $36.65   4.584%   $38.33   $38.33   $38.35   $38.30  
  7 purchase totals = $1,193.25       $1,277.8652   $1,277.87   $1,277.90   $1,277.80