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 $33.92   12.582%   $38.1878   $38.19   $38.20   $38.20  
  Purchase #2 $163.95   13.426%   $185.9619   $185.96   $185.95   $186.00  
  Purchase #3 $29.35   12.223%   $32.9375   $32.94   $32.95   $32.90  
  Purchase #4 $229.42   4.518%   $239.7852   $239.79   $239.80   $239.80  
  Purchase #5 $8.57   6.865%   $9.1583   $9.16   $9.15   $9.20  
  Purchase #6 $187.25   8.805%   $203.7374   $203.74   $203.75   $203.70  
  Purchase #7 $129.65   11.844%   $145.0057   $145.01   $145.00   $145.00  
  7 purchase totals = $782.11       $854.7738   $854.79   $854.80   $854.80