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 $122.62   11.169%   $136.3154   $136.32   $136.30   $136.30  
  Purchase #2 $92.56   4.759%   $96.9649   $96.96   $96.95   $97.00  
  Purchase #3 $223.78   7.788%   $241.208   $241.21   $241.20   $241.20  
  Purchase #4 $116.50   8.472%   $126.3699   $126.37   $126.35   $126.40  
  Purchase #5 $190.97   6.413%   $203.2169   $203.22   $203.20   $203.20  
  Purchase #6 $159.56   13.279%   $180.748   $180.75   $180.75   $180.70  
  Purchase #7 $32.71   11.474%   $36.4631   $36.46   $36.45   $36.50  
  7 purchase totals = $938.70       $1,021.2862   $1,021.29   $1,021.20   $1,021.30