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 $107.56   13.457%   $122.0343   $122.03   $122.05   $122.00  
  Purchase #2 $188.88   8.362%   $204.6741   $204.67   $204.65   $204.70  
  Purchase #3 $68.98   9.759%   $75.7118   $75.71   $75.70   $75.70  
  Purchase #4 $85.27   5.685%   $90.1176   $90.12   $90.10   $90.10  
  Purchase #5 $47.95   8.155%   $51.8603   $51.86   $51.85   $51.90  
  Purchase #6 $201.27   5.329%   $211.9957   $212.00   $212.00   $212.00  
  Purchase #7 $55.80   10.255%   $61.5223   $61.52   $61.50   $61.50  
  7 purchase totals = $755.71       $817.9161   $817.91   $817.85   $817.90