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 $212.54   13.234%   $240.6675   $240.67   $240.65   $240.70  
  Purchase #2 $30.32   8.746%   $32.9718   $32.97   $32.95   $33.00  
  Purchase #3 $49.41   5.854%   $52.3025   $52.30   $52.30   $52.30  
  Purchase #4 $123.99   6.818%   $132.4436   $132.44   $132.45   $132.40  
  Purchase #5 $173.72   3.623%   $180.0139   $180.01   $180.00   $180.00  
  Purchase #6 $119.29   6.267%   $126.7659   $126.77   $126.75   $126.80  
  Purchase #7 $69.42   3.310%   $71.7178   $71.72   $71.70   $71.70  
  7 purchase totals = $778.69       $836.8830   $836.88   $836.80   $836.90