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 $8.91   10.473%   $9.8431   $9.84   $9.85   $9.80  
  Purchase #2 $118.83   7.482%   $127.7209   $127.72   $127.70   $127.70  
  Purchase #3 $225.74   6.510%   $240.4357   $240.44   $240.45   $240.40  
  Purchase #4 $243.43   12.139%   $272.98   $272.98   $273.00   $273.00  
  Purchase #5 $107.35   7.210%   $115.0899   $115.09   $115.10   $115.10  
  Purchase #6 $35.33   9.449%   $38.6683   $38.67   $38.65   $38.70  
  Purchase #7 $106.49   6.475%   $113.3852   $113.39   $113.40   $113.40  
  7 purchase totals = $846.08       $918.1231   $918.13   $918.15   $918.10