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 $138.51   7.814%   $149.3332   $149.33   $149.35   $149.30  
  Purchase #2 $60.53   4.577%   $63.3005   $63.30   $63.30   $63.30  
  Purchase #3 $34.84   13.782%   $39.6416   $39.64   $39.65   $39.60  
  Purchase #4 $235.31   11.663%   $262.7542   $262.75   $262.75   $262.80  
  Purchase #5 $250.85   5.713%   $265.1811   $265.18   $265.20   $265.20  
  Purchase #6 $206.26   13.428%   $233.9566   $233.96   $233.95   $234.00  
  Purchase #7 $171.95   11.817%   $192.2693   $192.27   $192.25   $192.30  
  7 purchase totals = $1,098.25       $1,206.4365   $1,206.43   $1,206.45   $1,206.50