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 $133.58   3.452%   $138.1912   $138.19   $138.20   $138.20  
  Purchase #2 $115.60   10.700%   $127.9692   $127.97   $127.95   $128.00  
  Purchase #3 $145.78   5.525%   $153.8343   $153.83   $153.85   $153.80  
  Purchase #4 $27.80   13.373%   $31.5177   $31.52   $31.50   $31.50  
  Purchase #5 $114.90   5.517%   $121.239   $121.24   $121.25   $121.20  
  Purchase #6 $99.84   8.573%   $108.3993   $108.40   $108.40   $108.40  
  Purchase #7 $8.51   13.312%   $9.6429   $9.64   $9.65   $9.60  
  7 purchase totals = $646.01       $690.7936   $690.79   $690.80   $690.70