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 $108.58   13.810%   $123.5749   $123.57   $123.55   $123.60  
  Purchase #2 $131.42   9.321%   $143.6697   $143.67   $143.65   $143.70  
  Purchase #3 $224.98   8.242%   $243.5229   $243.52   $243.50   $243.50  
  Purchase #4 $244.30   6.654%   $260.5557   $260.56   $260.55   $260.60  
  Purchase #5 $220.91   6.577%   $235.4393   $235.44   $235.45   $235.40  
  Purchase #6 $243.27   12.102%   $272.7105   $272.71   $272.70   $272.70  
  Purchase #7 $88.51   8.476%   $96.0121   $96.01   $96.00   $96.00  
  7 purchase totals = $1,261.97       $1,375.4851   $1,375.48   $1,375.40   $1,375.50