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 $127.93   8.217%   $138.442   $138.44   $138.45   $138.40  
  Purchase #2 $110.81   11.158%   $123.1742   $123.17   $123.15   $123.20  
  Purchase #3 $24.43   6.735%   $26.0754   $26.08   $26.10   $26.10  
  Purchase #4 $126.92   9.781%   $139.334   $139.33   $139.35   $139.30  
  Purchase #5 $160.37   13.659%   $182.2749   $182.27   $182.25   $182.30  
  Purchase #6 $155.92   13.759%   $177.373   $177.37   $177.35   $177.40  
  Purchase #7 $177.67   6.832%   $189.8084   $189.81   $189.80   $189.80  
  7 purchase totals = $884.05       $976.4819   $976.47   $976.45   $976.50