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 $12.34   3.765%   $12.8046   $12.80   $12.80   $12.80  
  Purchase #2 $18.26   5.689%   $19.2988   $19.30   $19.30   $19.30  
  Purchase #3 $122.25   5.710%   $129.2305   $129.23   $129.25   $129.20  
  Purchase #4 $130.29   11.464%   $145.2264   $145.23   $145.25   $145.20  
  Purchase #5 $172.76   6.436%   $183.8788   $183.88   $183.90   $183.90  
  Purchase #6 $160.26   4.359%   $167.2457   $167.25   $167.25   $167.20  
  Purchase #7 $40.74   10.285%   $44.9301   $44.93   $44.95   $44.90  
  7 purchase totals = $656.90       $702.6149   $702.62   $702.70   $702.50