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 $13.72   8.846%   $14.9337   $14.93   $14.95   $14.90  
  Purchase #2 $10.63   3.646%   $11.0176   $11.02   $11.00   $11.00  
  Purchase #3 $113.35   12.280%   $127.2694   $127.27   $127.25   $127.30  
  Purchase #4 $110.50   7.256%   $118.5179   $118.52   $118.50   $118.50  
  Purchase #5 $189.26   10.606%   $209.3329   $209.33   $209.35   $209.30  
  Purchase #6 $116.72   10.628%   $129.125   $129.13   $129.15   $129.10  
  Purchase #7 $32.73   3.250%   $33.7937   $33.79   $33.80   $33.80  
  7 purchase totals = $586.91       $643.9902   $643.99   $644.00   $643.90