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 $92.38   9.150%   $100.8328   $100.83   $100.85   $100.80  
  Purchase #2 $80.75   4.515%   $84.3959   $84.40   $84.40   $84.40  
  Purchase #3 $9.92   12.824%   $11.1921   $11.19   $11.20   $11.20  
  Purchase #4 $6.59   5.711%   $6.9664   $6.97   $6.95   $7.00  
  Purchase #5 $45.87   9.740%   $50.3377   $50.34   $50.35   $50.30  
  Purchase #6 $91.63   9.231%   $100.0884   $100.09   $100.10   $100.10  
  Purchase #7 $139.83   12.249%   $156.9578   $156.96   $156.95   $157.00  
  7 purchase totals = $466.97       $510.7711   $510.78   $510.80   $510.80