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 $77.90   3.774%   $80.8399   $80.84   $80.85   $80.80  
  Purchase #2 $38.79   3.460%   $40.1321   $40.13   $40.15   $40.10  
  Purchase #3 $227.98   3.552%   $236.0778   $236.08   $236.10   $236.10  
  Purchase #4 $205.27   6.707%   $219.0375   $219.04   $219.05   $219.00  
  Purchase #5 $230.82   12.836%   $260.4481   $260.45   $260.45   $260.40  
  Purchase #6 $236.90   8.200%   $256.3258   $256.33   $256.35   $256.30  
  Purchase #7 $188.42   5.171%   $198.1632   $198.16   $198.15   $198.20  
  7 purchase totals = $1,206.08       $1,291.0244   $1,291.03   $1,291.10   $1,290.90