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 $20.43   6.799%   $21.819   $21.82   $21.80   $21.80  
  Purchase #2 $230.67   4.409%   $240.8402   $240.84   $240.85   $240.80  
  Purchase #3 $51.60   6.433%   $54.9194   $54.92   $54.90   $54.90  
  Purchase #4 $208.57   3.720%   $216.3288   $216.33   $216.35   $216.30  
  Purchase #5 $237.48   12.619%   $267.4476   $267.45   $267.45   $267.40  
  Purchase #6 $6.62   7.532%   $7.1186   $7.12   $7.10   $7.10  
  Purchase #7 $62.40   11.726%   $69.717   $69.72   $69.70   $69.70  
  7 purchase totals = $817.77       $878.1906   $878.20   $878.15   $878.00