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 $29.60   7.404%   $31.7916   $31.79   $31.80   $31.80  
  Purchase #2 $176.75   12.452%   $198.7589   $198.76   $198.75   $198.80  
  Purchase #3 $142.70   5.287%   $150.2445   $150.24   $150.25   $150.20  
  Purchase #4 $88.26   11.342%   $98.2704   $98.27   $98.25   $98.30  
  Purchase #5 $60.27   10.795%   $66.7761   $66.78   $66.80   $66.80  
  Purchase #6 $40.72   7.673%   $43.8444   $43.84   $43.85   $43.80  
  Purchase #7 $22.63   12.174%   $25.385   $25.39   $25.40   $25.40  
  7 purchase totals = $560.93       $615.0709   $615.07   $615.10   $615.10