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 $226.54   12.321%   $254.452   $254.45   $254.45   $254.50  
  Purchase #2 $110.54   10.286%   $121.9101   $121.91   $121.90   $121.90  
  Purchase #3 $60.66   7.490%   $65.2034   $65.20   $65.20   $65.20  
  Purchase #4 $197.66   8.205%   $213.878   $213.88   $213.90   $213.90  
  Purchase #5 $37.92   9.696%   $41.5967   $41.60   $41.60   $41.60  
  Purchase #6 $68.58   9.409%   $75.0327   $75.03   $75.05   $75.00  
  Purchase #7 $202.34   8.857%   $220.2613   $220.26   $220.25   $220.30  
  7 purchase totals = $904.24       $992.3342   $992.33   $992.35   $992.40