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 $25.97   3.171%   $26.7935   $26.79   $26.80   $26.80  
  Purchase #2 $246.48   11.756%   $275.4562   $275.46   $275.45   $275.50  
  Purchase #3 $148.48   3.221%   $153.2625   $153.26   $153.25   $153.30  
  Purchase #4 $137.70   9.667%   $151.0115   $151.01   $151.00   $151.00  
  Purchase #5 $116.61   7.410%   $125.2508   $125.25   $125.25   $125.30  
  Purchase #6 $163.79   5.181%   $172.276   $172.28   $172.30   $172.30  
  Purchase #7 $50.84   8.650%   $55.2377   $55.24   $55.25   $55.20  
  7 purchase totals = $889.87       $959.2882   $959.29   $959.30   $959.40