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 $240.48   12.383%   $270.2586   $270.26   $270.25   $270.30  
  Purchase #2 $108.45   9.675%   $118.9425   $118.94   $118.95   $118.90  
  Purchase #3 $116.98   10.614%   $129.3963   $129.40   $129.40   $129.40  
  Purchase #4 $246.40   11.763%   $275.384   $275.38   $275.40   $275.40  
  Purchase #5 $174.45   10.729%   $193.1667   $193.17   $193.15   $193.20  
  Purchase #6 $102.51   3.529%   $106.1276   $106.13   $106.15   $106.10  
  Purchase #7 $212.79   11.824%   $237.9503   $237.95   $237.95   $238.00  
  7 purchase totals = $1,202.06       $1,331.2260   $1,331.23   $1,331.25   $1,331.30