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 $19.97   5.561%   $21.0805   $21.08   $21.10   $21.10  
  Purchase #2 $125.87   8.306%   $136.3248   $136.32   $136.30   $136.30  
  Purchase #3 $190.48   9.201%   $208.0061   $208.01   $208.00   $208.00  
  Purchase #4 $160.33   6.644%   $170.9823   $170.98   $171.00   $171.00  
  Purchase #5 $18.92   5.790%   $20.0155   $20.02   $20.00   $20.00  
  Purchase #6 $170.57   5.747%   $180.3727   $180.37   $180.35   $180.40  
  Purchase #7 $129.30   5.688%   $136.6546   $136.65   $136.65   $136.70  
  7 purchase totals = $815.44       $873.4365   $873.43   $873.40   $873.50