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 $90.38   9.635%   $99.0881   $99.09   $99.10   $99.10  
  Purchase #2 $233.96   9.414%   $255.985   $255.99   $256.00   $256.00  
  Purchase #3 $27.91   10.864%   $30.9421   $30.94   $30.95   $30.90  
  Purchase #4 $235.49   3.168%   $242.9503   $242.95   $242.95   $243.00  
  Purchase #5 $171.96   10.870%   $190.6521   $190.65   $190.65   $190.70  
  Purchase #6 $215.50   7.270%   $231.1669   $231.17   $231.15   $231.20  
  Purchase #7 $25.40   11.361%   $28.2857   $28.29   $28.30   $28.30  
  7 purchase totals = $1,000.60       $1,079.0702   $1,079.08   $1,079.10   $1,079.20