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 $121.49   11.137%   $135.0203   $135.02   $135.00   $135.00  
  Purchase #2 $159.69   11.668%   $178.3226   $178.32   $178.30   $178.30  
  Purchase #3 $101.85   6.311%   $108.2778   $108.28   $108.30   $108.30  
  Purchase #4 $126.72   6.280%   $134.678   $134.68   $134.70   $134.70  
  Purchase #5 $241.93   4.665%   $253.216   $253.22   $253.20   $253.20  
  Purchase #6 $118.42   8.529%   $128.52   $128.52   $128.50   $128.50  
  Purchase #7 $35.79   7.160%   $38.3526   $38.35   $38.35   $38.40  
  7 purchase totals = $905.89       $976.3873   $976.39   $976.35   $976.40