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 $20.74   3.125%   $21.3881   $21.39   $21.40   $21.40  
  Purchase #2 $151.46   12.527%   $170.4334   $170.43   $170.45   $170.40  
  Purchase #3 $6.65   9.792%   $7.3012   $7.30   $7.30   $7.30  
  Purchase #4 $133.57   10.310%   $147.3411   $147.34   $147.35   $147.30  
  Purchase #5 $123.33   5.506%   $130.1205   $130.12   $130.10   $130.10  
  Purchase #6 $141.41   11.276%   $157.3554   $157.36   $157.35   $157.40  
  Purchase #7 $165.30   4.636%   $172.9633   $172.96   $172.95   $173.00  
  7 purchase totals = $742.46       $806.9030   $806.90   $806.90   $806.90