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 $172.91   9.837%   $189.9192   $189.92   $189.90   $189.90  
  Purchase #2 $103.77   13.734%   $118.0218   $118.02   $118.00   $118.00  
  Purchase #3 $240.99   13.719%   $274.0514   $274.05   $274.05   $274.10  
  Purchase #4 $158.45   7.321%   $170.0501   $170.05   $170.05   $170.10  
  Purchase #5 $162.60   5.478%   $171.5072   $171.51   $171.50   $171.50  
  Purchase #6 $52.80   10.543%   $58.3667   $58.37   $58.35   $58.40  
  Purchase #7 $196.35   6.661%   $209.4289   $209.43   $209.45   $209.40  
  7 purchase totals = $1,087.87       $1,191.3453   $1,191.35   $1,191.30   $1,191.40